diff --git a/benchmark/benchmarks.js b/benchmark/benchmarks.js
index 6987675..f492190 100644
--- a/benchmark/benchmarks.js
+++ b/benchmark/benchmarks.js
@@ -1,6 +1,6 @@
 "use strict";
 
-var benchmarks = [
+let benchmarks = [
   {
     id:          "json",
     title:       "JSON",
diff --git a/benchmark/index.js b/benchmark/index.js
index 97e54e5..cce8fcb 100644
--- a/benchmark/index.js
+++ b/benchmark/index.js
@@ -1,6 +1,6 @@
 /* eslint-env browser, jquery */
 
-var benchmarks = require("./benchmarks.js"),
+let benchmarks = require("./benchmarks.js"),
     Runner     = require("./runner.js");
 
 $("#run").click(function() {
@@ -8,7 +8,7 @@ $("#run").click(function() {
 
   /* Results Table Manipulation */
 
-  var resultsTable = $("#results-table");
+  let resultsTable = $("#results-table");
 
   function appendHeading(heading) {
     resultsTable.append(
@@ -17,8 +17,8 @@ $("#run").click(function() {
   }
 
   function appendResult(klass, title, url, inputSize, parseTime) {
-    var KB      = 1024,
-        MS_IN_S = 1000;
+    const KB      = 1024,
+          MS_IN_S = 1000;
 
     resultsTable.append(
         "<tr class='" + klass + "'>"
@@ -62,7 +62,7 @@ $("#run").click(function() {
    *   2. To minimize random errors.
    */
 
-  var runCount = parseInt($("#run-count").val(), 10),
+  let runCount = parseInt($("#run-count").val(), 10),
       options  = {
         cache:    $("#cache").is(":checked"),
         optimize: $("#optimize").val()
diff --git a/benchmark/run b/benchmark/run
index 7896f74..ee67203 100755
--- a/benchmark/run
+++ b/benchmark/run
@@ -5,16 +5,16 @@
 
 "use strict";
 
-var fs  = require("fs");
+let fs  = require("fs");
 
-var benchmarks = require("./benchmarks.js");
-var Runner = require("./runner.js");
+let benchmarks = require("./benchmarks.js");
+let Runner = require("./runner.js");
 
 /* Results Table Manipulation */
 
 function dup(text, count) {
-  var result = "";
-  for (var i = 1; i <= count; i++) {
+  let result = "";
+  for (let i = 1; i <= count; i++) {
     result += text;
   }
   return result;
@@ -29,7 +29,7 @@ function padRight(text, length) {
 }
 
 function center(text, length) {
-  var padLength = (length - text.length) / 2;
+  let padLength = (length - text.length) / 2;
   return dup(" ", Math.floor(padLength))
     + text
     + dup(" ", Math.ceil(padLength));
@@ -47,8 +47,8 @@ function writeHeading(heading) {
 }
 
 function writeResult(title, inputSize, parseTime) {
-  var KB      = 1024;
-  var MS_IN_S = 1000;
+  const KB      = 1024;
+  const MS_IN_S = 1000;
 
   console.log("│ "
     + padRight(title, 35)
@@ -99,7 +99,7 @@ function abort(message) {
 
 /* Arguments */
 
-var args = process.argv.slice(2); // Trim "node" and the script path.
+let args = process.argv.slice(2); // Trim "node" and the script path.
 
 function isOption(arg) {
   return (/^-/).test(arg);
@@ -111,8 +111,8 @@ function nextArg() {
 
 /* Main */
 
-var runCount = 10;
-var options = {
+let runCount = 10;
+let options = {
   cache:    false,
   optimize: "speed"
 };
diff --git a/benchmark/runner.js b/benchmark/runner.js
index 5090619..b28f93a 100644
--- a/benchmark/runner.js
+++ b/benchmark/runner.js
@@ -2,14 +2,14 @@
 
 "use strict";
 
-var peg = require("../lib/peg");
+let peg = require("../lib/peg");
 
-var Runner = {
+let Runner = {
   run: function(benchmarks, runCount, options, callbacks) {
 
     /* Queue */
 
-    var Q = {
+    let Q = {
           functions: [],
 
           add: function(f) {
@@ -44,7 +44,7 @@ var Runner = {
      * of the |state| object.
      */
 
-    var state = {};
+    let state = {};
 
     function initialize() {
       callbacks.start();
@@ -68,7 +68,7 @@ var Runner = {
 
     function testRunner(benchmark, test) {
       return function() {
-        var input, parseTime, averageParseTime, i, t;
+        let input, parseTime, averageParseTime, i, t;
 
         callbacks.testStart(benchmark, test);
 
diff --git a/benchmark/server b/benchmark/server
index 865a2fe..1125531 100755
--- a/benchmark/server
+++ b/benchmark/server
@@ -9,20 +9,20 @@
  * served to the browser.
  */
 
-var express    = require("express"),
+let express    = require("express"),
     logger     = require("morgan"),
     glob       = require("glob"),
     browserify = require("browserify"),
     babelify   = require("babelify");
 
-var app = express();
+let app = express();
 
 app.use(logger("dev"));
 app.use(express.static(__dirname));
 app.use("/examples", express.static(__dirname + "/../examples"));
 
 app.get("/bundle.js", function(req, res) {
-  var files = glob.sync(__dirname + "/**/*.js", {
+  let files = glob.sync(__dirname + "/**/*.js", {
     ignore: __dirname + "/vendor/**/*"
   });
 
diff --git a/bin/pegjs b/bin/pegjs
index 6ea1ec1..7a624b0 100755
--- a/bin/pegjs
+++ b/bin/pegjs
@@ -2,9 +2,9 @@
 
 "use strict";
 
-var fs      = require("fs");
-var path    = require("path");
-var peg     = require("../lib/peg");
+let fs      = require("fs");
+let path    = require("path");
+let peg     = require("../lib/peg");
 
 /* Helpers */
 
@@ -56,7 +56,7 @@ function abort(message) {
 }
 
 function addExtraOptions(options, json) {
-  var extraOptions;
+  let extraOptions;
 
   try {
     extraOptions = JSON.parse(json);
@@ -69,7 +69,7 @@ function addExtraOptions(options, json) {
     abort("The JSON with extra options has to represent an object.");
   }
 
-  for (var key in extraOptions) {
+  for (let key in extraOptions) {
     if (extraOptions.hasOwnProperty(key)) {
       options[key] = extraOptions[key];
     }
@@ -86,7 +86,7 @@ function trim(s) {
 
 /* Arguments */
 
-var args = process.argv.slice(2); // Trim "node" and the script path.
+let args = process.argv.slice(2); // Trim "node" and the script path.
 
 function isOption(arg) {
   return (/^-.+/).test(arg);
@@ -99,17 +99,17 @@ function nextArg() {
 /* Files */
 
 function readStream(inputStream, callback) {
-  var input = "";
+  let input = "";
   inputStream.on("data", function(data) { input += data; });
   inputStream.on("end", function() { callback(input); });
 }
 
 /* Main */
 
-var inputFile = null;
-var outputFile = null;
+let inputFile = null;
+let outputFile = null;
 
-var options = {
+let options = {
   cache:        false,
   dependencies: {},
   exportVar:    null,
@@ -121,6 +121,8 @@ var options = {
 };
 
 while (args.length > 0 && isOption(args[0])) {
+  let json, id, mod;
+
   switch (args[0]) {
     case "--allowed-start-rules":
       nextArg();
@@ -143,7 +145,7 @@ while (args.length > 0 && isOption(args[0])) {
         abort("Missing parameter of the -d/--dependency option.");
       }
       if (args[0].indexOf(":") !== -1) {
-        var parts = args[0].split(":");
+        let parts = args[0].split(":");
         options.dependencies[parts[0]] = parts[1];
       } else {
         options.dependencies[args[0]] = args[0];
@@ -173,7 +175,7 @@ while (args.length > 0 && isOption(args[0])) {
         abort("Missing parameter of the --extra-options-file option.");
       }
       try {
-        var json = fs.readFileSync(args[0]);
+        json = fs.readFileSync(args[0]);
       } catch(e) {
         abort("Can't read from file \"" + args[0] + "\".");
       }
@@ -223,8 +225,8 @@ while (args.length > 0 && isOption(args[0])) {
       if (args.length === 0) {
         abort("Missing parameter of the --plugin option.");
       }
-      var id = /^(\.\/|\.\.\/)/.test(args[0]) ? path.resolve(args[0]) : args[0];
-      var mod;
+      id = /^(\.\/|\.\.\/)/.test(args[0]) ? path.resolve(args[0]) : args[0];
+      mod;
       try {
         mod = require(id);
       } catch (e) {
@@ -267,8 +269,8 @@ if (options.exportVar !== null) {
   }
 }
 
-var inputStream;
-var outputStream;
+let inputStream;
+let outputStream;
 
 switch (args.length) {
   case 0:
@@ -311,7 +313,7 @@ if (outputFile === "-") {
 }
 
 readStream(inputStream, function(input) {
-  var source;
+  let source;
 
   try {
     source = peg.generate(input, options);
diff --git a/lib/compiler/asts.js b/lib/compiler/asts.js
index 0b1ce44..b24979f 100644
--- a/lib/compiler/asts.js
+++ b/lib/compiler/asts.js
@@ -1,10 +1,10 @@
 "use strict";
 
-var arrays  = require("../utils/arrays"),
+let arrays  = require("../utils/arrays"),
     visitor = require("./visitor");
 
 /* AST utilities. */
-var asts = {
+let asts = {
   findRule: function(ast, name) {
     return arrays.find(ast.rules, function(r) { return r.name === name; });
   },
@@ -21,7 +21,7 @@ var asts = {
       return consumes(node.expression);
     }
 
-    var consumes = visitor.build({
+    let consumes = visitor.build({
       rule:  consumesExpression,
       named: consumesExpression,
 
diff --git a/lib/compiler/index.js b/lib/compiler/index.js
index 0607f8e..e4a0d57 100644
--- a/lib/compiler/index.js
+++ b/lib/compiler/index.js
@@ -1,8 +1,8 @@
 "use strict";
 
-var objects = require("../utils/objects");
+let objects = require("../utils/objects");
 
-var compiler = {
+let compiler = {
   /*
    * AST node visitor builder. Useful mainly for plugins which manipulate the
    * AST.
@@ -42,7 +42,7 @@ var compiler = {
   compile: function(ast, passes, options) {
     options = options !== undefined ? options : {};
 
-    var stage;
+    let stage;
 
     options = objects.clone(options);
     objects.defaults(options, {
diff --git a/lib/compiler/js.js b/lib/compiler/js.js
index 07f5ab0..5f91bc4 100644
--- a/lib/compiler/js.js
+++ b/lib/compiler/js.js
@@ -3,7 +3,7 @@
 function hex(ch) { return ch.charCodeAt(0).toString(16).toUpperCase(); }
 
 /* JavaScript code generation helpers. */
-var js = {
+let js = {
   stringEscape: function(s) {
     /*
      * ECMA-262, 5th ed., 7.8.4: All characters may appear literally in a string
diff --git a/lib/compiler/opcodes.js b/lib/compiler/opcodes.js
index 4c52008..da5ff4b 100644
--- a/lib/compiler/opcodes.js
+++ b/lib/compiler/opcodes.js
@@ -1,7 +1,7 @@
 "use strict";
 
 /* Bytecode instruction opcodes. */
-var opcodes = {
+let opcodes = {
   /* Stack Manipulation */
 
   PUSH:             0,    // PUSH c
diff --git a/lib/compiler/passes/generate-bytecode.js b/lib/compiler/passes/generate-bytecode.js
index 934df96..75d90c5 100644
--- a/lib/compiler/passes/generate-bytecode.js
+++ b/lib/compiler/passes/generate-bytecode.js
@@ -1,6 +1,6 @@
 "use strict";
 
-var arrays  = require("../../utils/arrays"),
+let arrays  = require("../../utils/arrays"),
     objects = require("../../utils/objects"),
     asts    = require("../asts"),
     visitor = require("../visitor"),
@@ -191,10 +191,10 @@ var arrays  = require("../../utils/arrays"),
  *        silentFails--;
  */
 function generateBytecode(ast) {
-  var consts = [];
+  let consts = [];
 
   function addConst(value) {
-    var index = arrays.indexOf(consts, value);
+    let index = arrays.indexOf(consts, value);
 
     return index === -1 ? consts.push(value) - 1 : index;
   }
@@ -222,7 +222,7 @@ function generateBytecode(ast) {
   }
 
   function buildCall(functionIndex, delta, env, sp) {
-    var params = objects.values(env).map(function(p) { return sp - p; });
+    let params = objects.values(env).map(function(p) { return sp - p; });
 
     return [op.CALL, functionIndex, delta, params.length].concat(params);
   }
@@ -254,7 +254,7 @@ function generateBytecode(ast) {
   }
 
   function buildSemanticPredicate(code, negative, context) {
-    var functionIndex = addFunctionConst(Object.keys(context.env), code);
+    let functionIndex = addFunctionConst(Object.keys(context.env), code);
 
     return buildSequence(
       [op.UPDATE_SAVED_POS],
@@ -280,7 +280,7 @@ function generateBytecode(ast) {
     );
   }
 
-  var generate = visitor.build({
+  let generate = visitor.build({
     grammar: function(node) {
       node.rules.forEach(generate);
 
@@ -296,7 +296,7 @@ function generateBytecode(ast) {
     },
 
     named: function(node, context) {
-      var nameIndex = addConst(
+      let nameIndex = addConst(
         'peg$otherExpectation("' + js.stringEscape(node.name) + '")'
       );
 
@@ -339,7 +339,7 @@ function generateBytecode(ast) {
     },
 
     action: function(node, context) {
-      var env            = objects.clone(context.env),
+      let env            = objects.clone(context.env),
           emitCall       = node.expression.type !== "sequence"
                         || node.expression.elements.length === 0,
           expressionCode = generate(node.expression, {
@@ -368,7 +368,7 @@ function generateBytecode(ast) {
 
     sequence: function(node, context) {
       function buildElementsCode(elements, context) {
-        var processedCount, functionIndex;
+        let processedCount, functionIndex;
 
         if (elements.length > 0) {
           processedCount = node.elements.length - elements.slice(1).length;
@@ -427,7 +427,7 @@ function generateBytecode(ast) {
     },
 
     labeled: function(node, context) {
-      var env = objects.clone(context.env);
+      let env = objects.clone(context.env);
 
       context.env[node.label] = context.sp + 1;
 
@@ -478,7 +478,7 @@ function generateBytecode(ast) {
     },
 
     zero_or_more: function(node, context) {
-      var expressionCode = generate(node.expression, {
+      let expressionCode = generate(node.expression, {
             sp:     context.sp + 1,
             env:    objects.clone(context.env),
             action: null
@@ -493,7 +493,7 @@ function generateBytecode(ast) {
     },
 
     one_or_more: function(node, context) {
-      var expressionCode = generate(node.expression, {
+      let expressionCode = generate(node.expression, {
             sp:     context.sp + 1,
             env:    objects.clone(context.env),
             action: null
@@ -531,7 +531,7 @@ function generateBytecode(ast) {
     },
 
     literal: function(node) {
-      var stringIndex, expectedIndex;
+      let stringIndex, expectedIndex;
 
       if (node.value.length > 0) {
         stringIndex = addConst('"'
@@ -569,7 +569,7 @@ function generateBytecode(ast) {
     },
 
     "class": function(node) {
-      var regexp        = '/^['
+      let regexp        = '/^['
             + (node.inverted ? '^' : '')
             + node.parts.map(function(part) {
                 return Array.isArray(part)
@@ -603,7 +603,7 @@ function generateBytecode(ast) {
     },
 
     any: function() {
-      var expectedIndex = addConst('peg$anyExpectation()');
+      let expectedIndex = addConst('peg$anyExpectation()');
 
       return buildCondition(
         [op.MATCH_ANY],
diff --git a/lib/compiler/passes/generate-js.js b/lib/compiler/passes/generate-js.js
index 4108579..41c02f2 100644
--- a/lib/compiler/passes/generate-js.js
+++ b/lib/compiler/passes/generate-js.js
@@ -1,6 +1,6 @@
 "use strict";
 
-var arrays  = require("../../utils/arrays"),
+let arrays  = require("../../utils/arrays"),
     objects = require("../../utils/objects"),
     asts    = require("../asts"),
     op      = require("../opcodes"),
@@ -38,7 +38,7 @@ function generateJS(ast, options) {
   }
 
   function generateRuleHeader(ruleNameCode, ruleIndexCode) {
-    var parts = [];
+    let parts = [];
 
     parts.push('');
 
@@ -94,7 +94,7 @@ function generateJS(ast, options) {
   }
 
   function generateRuleFooter(ruleNameCode, resultCode) {
-    var parts = [];
+    let parts = [];
 
     if (options.cache) {
       parts.push([
@@ -132,10 +132,10 @@ function generateJS(ast, options) {
   }
 
   function generateInterpreter() {
-    var parts = [];
+    let parts = [];
 
     function generateCondition(cond, argsLength) {
-      var baseLength      = argsLength + 3,
+      let baseLength      = argsLength + 3,
           thenLengthCode = 'bc[ip + ' + (baseLength - 2) + ']',
           elseLengthCode = 'bc[ip + ' + (baseLength - 1) + ']';
 
@@ -156,7 +156,7 @@ function generateJS(ast, options) {
     }
 
     function generateLoop(cond) {
-      var baseLength     = 2,
+      let baseLength     = 2,
           bodyLengthCode = 'bc[ip + ' + (baseLength - 1) + ']';
 
       return [
@@ -175,7 +175,7 @@ function generateJS(ast, options) {
     }
 
     function generateCall() {
-      var baseLength       = 4,
+      let baseLength       = 4,
           paramsLengthCode = 'bc[ip + ' + (baseLength - 1) + ']';
 
       return [
@@ -410,17 +410,17 @@ function generateJS(ast, options) {
   }
 
   function generateRuleFunction(rule) {
-    var parts = [], code;
+    let parts = [], code;
 
     function c(i) { return "peg$c" + i; } // |consts[i]| of the abstract machine
     function s(i) { return "s"     + i; } // |stack[i]| of the abstract machine
 
-    var stack = {
+    let stack = {
           sp:    -1,
           maxSp: -1,
 
           push: function(exprCode) {
-            var code = s(++this.sp) + ' = ' + exprCode + ';';
+            let code = s(++this.sp) + ' = ' + exprCode + ';';
 
             if (this.sp > this.maxSp) { this.maxSp = this.sp; }
 
@@ -428,7 +428,7 @@ function generateJS(ast, options) {
           },
 
           pop: function(n) {
-            var values;
+            let values;
 
             if (n === undefined) {
               return s(this.sp--);
@@ -450,13 +450,13 @@ function generateJS(ast, options) {
         };
 
     function compile(bc) {
-      var ip    = 0,
+      let ip    = 0,
           end   = bc.length,
           parts = [],
           value;
 
       function compileCondition(cond, argCount) {
-        var baseLength = argCount + 3,
+        let baseLength = argCount + 3,
             thenLength = bc[ip + baseLength - 2],
             elseLength = bc[ip + baseLength - 1],
             baseSp     = stack.sp,
@@ -490,7 +490,7 @@ function generateJS(ast, options) {
       }
 
       function compileLoop(cond) {
-        var baseLength = 2,
+        let baseLength = 2,
             bodyLength = bc[ip + baseLength - 1],
             baseSp     = stack.sp,
             bodyCode, bodySp;
@@ -510,10 +510,10 @@ function generateJS(ast, options) {
       }
 
       function compileCall() {
-        var baseLength   = 4,
+        let baseLength   = 4,
             paramsLength = bc[ip + baseLength - 1];
 
-        var value = c(bc[ip + 1]) + '('
+        let value = c(bc[ip + 1]) + '('
               + bc.slice(ip + baseLength, ip + baseLength + paramsLength).map(
                   function(p) { return stack.index(p); }
                 ).join(', ')
@@ -745,7 +745,7 @@ function generateJS(ast, options) {
   }
 
   function generateToplevel() {
-    var parts = [],
+    let parts = [],
         startRuleIndices,   startRuleIndex,
         startRuleFunctions, startRuleFunction,
         ruleNames;
@@ -1229,7 +1229,7 @@ function generateJS(ast, options) {
           ].join('\n');
     }
 
-    var generators = {
+    let generators = {
       bare: function() {
         return [
           generateGeneratedByComment(),
@@ -1244,7 +1244,7 @@ function generateJS(ast, options) {
       },
 
       commonjs: function() {
-        var parts          = [],
+        let parts          = [],
             dependencyVars = Object.keys(options.dependencies),
             requires       = dependencyVars.map(
               function(variable) {
@@ -1278,7 +1278,7 @@ function generateJS(ast, options) {
       },
 
       amd: function() {
-        var dependencyIds  = objects.values(options.dependencies),
+        let dependencyIds  = objects.values(options.dependencies),
             dependencyVars = Object.keys(options.dependencies),
             dependencies   = '['
               + dependencyIds.map(
@@ -1315,7 +1315,7 @@ function generateJS(ast, options) {
       },
 
       umd: function() {
-        var parts          = [],
+        let parts          = [],
             dependencyIds  = objects.values(options.dependencies),
             dependencyVars = Object.keys(options.dependencies),
             dependencies   = '['
diff --git a/lib/compiler/passes/remove-proxy-rules.js b/lib/compiler/passes/remove-proxy-rules.js
index d90c36f..f8036c0 100644
--- a/lib/compiler/passes/remove-proxy-rules.js
+++ b/lib/compiler/passes/remove-proxy-rules.js
@@ -1,6 +1,6 @@
 "use strict";
 
-var arrays  = require("../../utils/arrays"),
+let arrays  = require("../../utils/arrays"),
     visitor = require("../visitor");
 
 /*
@@ -12,7 +12,7 @@ function removeProxyRules(ast, options) {
   }
 
   function replaceRuleRefs(ast, from, to) {
-    var replace = visitor.build({
+    let replace = visitor.build({
       rule_ref: function(node) {
         if (node.name === from) {
           node.name = to;
@@ -23,7 +23,7 @@ function removeProxyRules(ast, options) {
     replace(ast);
   }
 
-  var indices = [];
+  let indices = [];
 
   ast.rules.forEach(function(rule, i) {
     if (isProxyRule(rule)) {
diff --git a/lib/compiler/passes/report-duplicate-labels.js b/lib/compiler/passes/report-duplicate-labels.js
index c42b57e..f13659f 100644
--- a/lib/compiler/passes/report-duplicate-labels.js
+++ b/lib/compiler/passes/report-duplicate-labels.js
@@ -1,6 +1,6 @@
 "use strict";
 
-var GrammarError = require("../../grammar-error"),
+let GrammarError = require("../../grammar-error"),
     objects      = require("../../utils/objects"),
     visitor      = require("../visitor");
 
@@ -10,7 +10,7 @@ function reportDuplicateLabels(ast) {
     check(node.expression, objects.clone(env));
   }
 
-  var check = visitor.build({
+  let check = visitor.build({
     rule: function(node) {
       check(node.expression, { });
     },
diff --git a/lib/compiler/passes/report-duplicate-rules.js b/lib/compiler/passes/report-duplicate-rules.js
index c621d37..47714dd 100644
--- a/lib/compiler/passes/report-duplicate-rules.js
+++ b/lib/compiler/passes/report-duplicate-rules.js
@@ -1,13 +1,13 @@
 "use strict";
 
-var GrammarError = require("../../grammar-error"),
+let GrammarError = require("../../grammar-error"),
     visitor      = require("../visitor");
 
 /* Checks that each rule is defined only once. */
 function reportDuplicateRules(ast) {
-  var rules = {};
+  let rules = {};
 
-  var check = visitor.build({
+  let check = visitor.build({
     rule: function(node) {
       if (rules.hasOwnProperty(node.name)) {
         throw new GrammarError(
diff --git a/lib/compiler/passes/report-infinite-recursion.js b/lib/compiler/passes/report-infinite-recursion.js
index 6623a8b..fc733d0 100644
--- a/lib/compiler/passes/report-infinite-recursion.js
+++ b/lib/compiler/passes/report-infinite-recursion.js
@@ -1,6 +1,6 @@
 "use strict";
 
-var arrays       = require("../../utils/arrays"),
+let arrays       = require("../../utils/arrays"),
     GrammarError = require("../../grammar-error"),
     asts         = require("../asts"),
     visitor      = require("../visitor");
@@ -18,9 +18,9 @@ var arrays       = require("../../utils/arrays"),
  * it can lead to left recursion.
  */
 function reportInfiniteRecursion(ast) {
-  var visitedRules = [];
+  let visitedRules = [];
 
-  var check = visitor.build({
+  let check = visitor.build({
     rule: function(node) {
       visitedRules.push(node.name);
       check(node.expression);
diff --git a/lib/compiler/passes/report-infinite-repetition.js b/lib/compiler/passes/report-infinite-repetition.js
index e6f12c0..2942b65 100644
--- a/lib/compiler/passes/report-infinite-repetition.js
+++ b/lib/compiler/passes/report-infinite-repetition.js
@@ -1,6 +1,6 @@
 "use strict";
 
-var GrammarError = require("../../grammar-error"),
+let GrammarError = require("../../grammar-error"),
     asts         = require("../asts"),
     visitor      = require("../visitor");
 
@@ -9,7 +9,7 @@ var GrammarError = require("../../grammar-error"),
  * grammar, which prevents infinite loops in the generated parser.
  */
 function reportInfiniteRepetition(ast) {
-  var check = visitor.build({
+  let check = visitor.build({
     zero_or_more: function(node) {
       if (!asts.alwaysConsumesOnSuccess(ast, node.expression)) {
         throw new GrammarError(
diff --git a/lib/compiler/passes/report-undefined-rules.js b/lib/compiler/passes/report-undefined-rules.js
index c71b03f..78b5d65 100644
--- a/lib/compiler/passes/report-undefined-rules.js
+++ b/lib/compiler/passes/report-undefined-rules.js
@@ -1,12 +1,12 @@
 "use strict";
 
-var GrammarError = require("../../grammar-error"),
+let GrammarError = require("../../grammar-error"),
     asts         = require("../asts"),
     visitor      = require("../visitor");
 
 /* Checks that all referenced rules exist. */
 function reportUndefinedRules(ast) {
-  var check = visitor.build({
+  let check = visitor.build({
     rule_ref: function(node) {
       if (!asts.findRule(ast, node.name)) {
         throw new GrammarError(
diff --git a/lib/compiler/visitor.js b/lib/compiler/visitor.js
index a7f764e..58545b0 100644
--- a/lib/compiler/visitor.js
+++ b/lib/compiler/visitor.js
@@ -1,9 +1,9 @@
 "use strict";
 
-var objects = require("../utils/objects");
+let objects = require("../utils/objects");
 
 /* Simple AST node visitor builder. */
-var visitor = {
+let visitor = {
   build: function(functions) {
     function visit(node) {
       return functions[node.type].apply(null, arguments);
@@ -12,14 +12,14 @@ var visitor = {
     function visitNop() { }
 
     function visitExpression(node) {
-      var extraArgs = Array.prototype.slice.call(arguments, 1);
+      let extraArgs = Array.prototype.slice.call(arguments, 1);
 
       visit.apply(null, [node.expression].concat(extraArgs));
     }
 
     function visitChildren(property) {
       return function(node) {
-        var extraArgs = Array.prototype.slice.call(arguments, 1);
+        let extraArgs = Array.prototype.slice.call(arguments, 1);
 
         node[property].forEach(function(child) {
           visit.apply(null, [child].concat(extraArgs));
@@ -27,9 +27,9 @@ var visitor = {
       };
     }
 
-    var DEFAULT_FUNCTIONS = {
+    const DEFAULT_FUNCTIONS = {
           grammar: function(node) {
-            var extraArgs = Array.prototype.slice.call(arguments, 1);
+            let extraArgs = Array.prototype.slice.call(arguments, 1);
 
             if (node.initializer) {
               visit.apply(null, [node.initializer].concat(extraArgs));
diff --git a/lib/grammar-error.js b/lib/grammar-error.js
index 758b8e9..99c8215 100644
--- a/lib/grammar-error.js
+++ b/lib/grammar-error.js
@@ -1,6 +1,6 @@
 "use strict";
 
-var classes = require("./utils/classes");
+let classes = require("./utils/classes");
 
 /* Thrown when the grammar contains an error. */
 function GrammarError(message, location) {
diff --git a/lib/parser.js b/lib/parser.js
index 5ba5bfc..b9edab9 100644
--- a/lib/parser.js
+++ b/lib/parser.js
@@ -4960,19 +4960,19 @@ function peg$parse(input, options) {
   }
 
 
-    var OPS_TO_PREFIXED_TYPES = {
+    const OPS_TO_PREFIXED_TYPES = {
       "$": "text",
       "&": "simple_and",
       "!": "simple_not"
     };
 
-    var OPS_TO_SUFFIXED_TYPES = {
+    const OPS_TO_SUFFIXED_TYPES = {
       "?": "optional",
       "*": "zero_or_more",
       "+": "one_or_more"
     };
 
-    var OPS_TO_SEMANTIC_PREDICATE_TYPES = {
+    const OPS_TO_SEMANTIC_PREDICATE_TYPES = {
       "&": "semantic_and",
       "!": "semantic_not"
     };
diff --git a/lib/peg.js b/lib/peg.js
index ee8027c..b95143d 100644
--- a/lib/peg.js
+++ b/lib/peg.js
@@ -1,8 +1,8 @@
 "use strict";
 
-var objects = require("./utils/objects");
+let objects = require("./utils/objects");
 
-var peg = {
+let peg = {
   /* PEG.js version (uses semantic versioning). */
   VERSION: "0.10.0",
 
@@ -25,7 +25,7 @@ var peg = {
     options = options !== undefined ? options : {};
 
     function convertPasses(passes) {
-      var converted = {}, stage;
+      let converted = {}, stage;
 
       for (stage in passes) {
         if (passes.hasOwnProperty(stage)) {
@@ -38,7 +38,7 @@ var peg = {
 
     options = objects.clone(options);
 
-    var plugins = "plugins" in options ? options.plugins : [],
+    let plugins = "plugins" in options ? options.plugins : [],
         config  = {
           parser: peg.parser,
           passes: convertPasses(peg.compiler.passes)
diff --git a/lib/utils/arrays.js b/lib/utils/arrays.js
index f22948f..15f8c0a 100644
--- a/lib/utils/arrays.js
+++ b/lib/utils/arrays.js
@@ -1,9 +1,9 @@
 "use strict";
 
 /* Array utilities. */
-var arrays = {
+let arrays = {
   range: function(start, stop) {
-    var length = stop - start,
+    let length = stop - start,
         result = new Array(length),
         i, j;
 
@@ -15,7 +15,7 @@ var arrays = {
   },
 
   find: function(array, valueOrPredicate) {
-    var length = array.length, i;
+    let length = array.length, i;
 
     if (typeof valueOrPredicate === "function") {
       for (i = 0; i < length; i++) {
@@ -33,7 +33,7 @@ var arrays = {
   },
 
   indexOf: function(array, valueOrPredicate) {
-    var length = array.length, i;
+    let length = array.length, i;
 
     if (typeof valueOrPredicate === "function") {
       for (i = 0; i < length; i++) {
diff --git a/lib/utils/classes.js b/lib/utils/classes.js
index 1ad305e..49c1c45 100644
--- a/lib/utils/classes.js
+++ b/lib/utils/classes.js
@@ -1,7 +1,7 @@
 "use strict";
 
 /* Class utilities */
-var classes = {
+let classes = {
   subclass: function(child, parent) {
     function ctor() { this.constructor = child; }
     ctor.prototype = parent.prototype;
diff --git a/lib/utils/objects.js b/lib/utils/objects.js
index 6973095..9f73876 100644
--- a/lib/utils/objects.js
+++ b/lib/utils/objects.js
@@ -1,9 +1,9 @@
 "use strict";
 
 /* Object utilities. */
-var objects = {
+let objects = {
   values: function(object) {
-    var result = [], key;
+    let result = [], key;
 
     for (key in object) {
       if (object.hasOwnProperty(key)) {
@@ -15,7 +15,7 @@ var objects = {
   },
 
   clone: function(object) {
-    var result = {}, key;
+    let result = {}, key;
 
     for (key in object) {
       if (object.hasOwnProperty(key)) {
@@ -27,7 +27,7 @@ var objects = {
   },
 
   defaults: function(object, defaults) {
-    var key;
+    let key;
 
     for (key in defaults) {
       if (defaults.hasOwnProperty(key)) {
diff --git a/spec/api/generated-parser-api.spec.js b/spec/api/generated-parser-api.spec.js
index 3ff1dcf..85a50b5 100644
--- a/spec/api/generated-parser-api.spec.js
+++ b/spec/api/generated-parser-api.spec.js
@@ -3,24 +3,24 @@
 
 "use strict";
 
-var peg = require("../../lib/peg");
+let peg = require("../../lib/peg");
 
 describe("generated parser API", function() {
   describe("parse", function() {
     it("parses input", function() {
-      var parser = peg.generate('start = "a"');
+      let parser = peg.generate('start = "a"');
 
       expect(parser.parse("a")).toBe("a");
     });
 
     it("throws an exception on syntax error", function() {
-      var parser = peg.generate('start = "a"');
+      let parser = peg.generate('start = "a"');
 
       expect(function() { parser.parse("b"); }).toThrow();
     });
 
     describe("start rule", function() {
-      var parser = peg.generate([
+      let parser = peg.generate([
             'a = "x" { return "a"; }',
             'b = "x" { return "b"; }',
             'c = "x" { return "c"; }'
@@ -49,7 +49,7 @@ describe("generated parser API", function() {
     });
 
     describe("tracing", function() {
-      var parser = peg.generate([
+      let parser = peg.generate([
             'start = a / b',
             'a = "a"',
             'b = "b"'
@@ -77,7 +77,7 @@ describe("generated parser API", function() {
       describe("custom tracers", function() {
         describe("trace", function() {
           it("receives tracing events", function() {
-            var tracer = jasmine.createSpyObj("tracer", ["trace"]);
+            let tracer = jasmine.createSpyObj("tracer", ["trace"]);
 
             parser.parse("b", { tracer: tracer });
 
@@ -137,7 +137,7 @@ describe("generated parser API", function() {
     });
 
     it("accepts custom options", function() {
-      var parser = peg.generate('start = "a"');
+      let parser = peg.generate('start = "a"');
 
       parser.parse("a", { foo: 42 });
     });
diff --git a/spec/api/pegjs-api.spec.js b/spec/api/pegjs-api.spec.js
index 9d02c66..1e2e69e 100644
--- a/spec/api/pegjs-api.spec.js
+++ b/spec/api/pegjs-api.spec.js
@@ -1,11 +1,11 @@
 "use strict";
 
-var peg = require("../../lib/peg");
+let peg = require("../../lib/peg");
 
 describe("PEG.js API", function() {
   describe("generate", function() {
     it("generates a parser", function() {
-      var parser = peg.generate('start = "a"');
+      let parser = peg.generate('start = "a"');
 
       expect(typeof parser).toBe("object");
       expect(parser.parse("a")).toBe("a");
@@ -20,7 +20,7 @@ describe("PEG.js API", function() {
     });
 
     describe("allowed start rules", function() {
-      var grammar = [
+      let grammar = [
             'a = "x"',
             'b = "x"',
             'c = "x"'
@@ -34,7 +34,7 @@ describe("PEG.js API", function() {
       describe("when optimizing for parsing speed", function() {
         describe("when |allowedStartRules| is not set", function() {
           it("generated parser can start only from the first rule", function() {
-            var parser = peg.generate(grammar, { optimize: "speed" });
+            let parser = peg.generate(grammar, { optimize: "speed" });
 
             expect(parser.parse("x", { startRule: "a" })).toBe("x");
             expect(
@@ -48,7 +48,7 @@ describe("PEG.js API", function() {
 
         describe("when |allowedStartRules| is set", function() {
           it("generated parser can start only from specified rules", function() {
-            var parser = peg.generate(grammar, {
+            let parser = peg.generate(grammar, {
               optimize:          "speed",
               allowedStartRules: ["b", "c"]
             });
@@ -65,7 +65,7 @@ describe("PEG.js API", function() {
       describe("when optimizing for code size", function() {
         describe("when |allowedStartRules| is not set", function() {
           it("generated parser can start only from the first rule", function() {
-            var parser = peg.generate(grammar, { optimize: "size" });
+            let parser = peg.generate(grammar, { optimize: "size" });
 
             expect(parser.parse("x", { startRule: "a" })).toBe("x");
             expect(
@@ -79,7 +79,7 @@ describe("PEG.js API", function() {
 
         describe("when |allowedStartRules| is set", function() {
           it("generated parser can start only from specified rules", function() {
-            var parser = peg.generate(grammar, {
+            let parser = peg.generate(grammar, {
               optimize:          "size",
               allowedStartRules: ["b", "c"]
             });
@@ -95,7 +95,7 @@ describe("PEG.js API", function() {
     });
 
     describe("intermediate results caching", function() {
-      var grammar = [
+      let grammar = [
             '{ var n = 0; }',
             'start = (a "b") / (a "c") { return n; }',
             'a     = "a" { n++; }'
@@ -103,7 +103,7 @@ describe("PEG.js API", function() {
 
       describe("when |cache| is not set", function() {
         it("generated parser doesn't cache intermediate parse results", function() {
-          var parser = peg.generate(grammar);
+          let parser = peg.generate(grammar);
 
           expect(parser.parse("ac")).toBe(2);
         });
@@ -111,7 +111,7 @@ describe("PEG.js API", function() {
 
       describe("when |cache| is set to |false|", function() {
         it("generated parser doesn't cache intermediate parse results", function() {
-          var parser = peg.generate(grammar, { cache: false });
+          let parser = peg.generate(grammar, { cache: false });
 
           expect(parser.parse("ac")).toBe(2);
         });
@@ -119,7 +119,7 @@ describe("PEG.js API", function() {
 
       describe("when |cache| is set to |true|", function() {
         it("generated parser caches intermediate parse results", function() {
-          var parser = peg.generate(grammar, { cache: true });
+          let parser = peg.generate(grammar, { cache: true });
 
           expect(parser.parse("ac")).toBe(1);
         });
@@ -127,11 +127,11 @@ describe("PEG.js API", function() {
     });
 
     describe("tracing", function() {
-      var grammar = 'start = "a"';
+      let grammar = 'start = "a"';
 
       describe("when |trace| is not set", function() {
         it("generated parser doesn't trace", function() {
-          var parser = peg.generate(grammar),
+          let parser = peg.generate(grammar),
               tracer = jasmine.createSpyObj("tracer", ["trace"]);
 
           parser.parse("a", { tracer: tracer });
@@ -142,7 +142,7 @@ describe("PEG.js API", function() {
 
       describe("when |trace| is set to |false|", function() {
         it("generated parser doesn't trace", function() {
-          var parser = peg.generate(grammar, { trace: false }),
+          let parser = peg.generate(grammar, { trace: false }),
               tracer = jasmine.createSpyObj("tracer", ["trace"]);
 
           parser.parse("a", { tracer: tracer });
@@ -153,7 +153,7 @@ describe("PEG.js API", function() {
 
       describe("when |trace| is set to |true|", function() {
         it("generated parser traces", function() {
-          var parser = peg.generate(grammar, { trace: true }),
+          let parser = peg.generate(grammar, { trace: true }),
               tracer = jasmine.createSpyObj("tracer", ["trace"]);
 
           parser.parse("a", { tracer: tracer });
@@ -169,11 +169,11 @@ describe("PEG.js API", function() {
      */
 
     describe("output", function() {
-      var grammar = 'start = "a"';
+      let grammar = 'start = "a"';
 
       describe("when |output| is not set", function() {
         it("returns generated parser object", function() {
-          var parser = peg.generate(grammar);
+          let parser = peg.generate(grammar);
 
           expect(typeof parser).toBe("object");
           expect(parser.parse("a")).toBe("a");
@@ -182,7 +182,7 @@ describe("PEG.js API", function() {
 
       describe("when |output| is set to |\"parser\"|", function() {
         it("returns generated parser object", function() {
-          var parser = peg.generate(grammar, { output: "parser" });
+          let parser = peg.generate(grammar, { output: "parser" });
 
           expect(typeof parser).toBe("object");
           expect(parser.parse("a")).toBe("a");
@@ -191,7 +191,7 @@ describe("PEG.js API", function() {
 
       describe("when |output| is set to |\"source\"|", function() {
         it("returns generated parser source code", function() {
-          var source = peg.generate(grammar, { output: "source" });
+          let source = peg.generate(grammar, { output: "source" });
 
           expect(typeof source).toBe("string");
           expect(eval(source).parse("a")).toBe("a");
diff --git a/spec/api/plugin-api.spec.js b/spec/api/plugin-api.spec.js
index d89356f..da0cd19 100644
--- a/spec/api/plugin-api.spec.js
+++ b/spec/api/plugin-api.spec.js
@@ -1,6 +1,6 @@
 "use strict";
 
-var peg = require("../../lib/peg");
+let peg = require("../../lib/peg");
 
 describe("plugin API", function() {
   beforeEach(function() {
@@ -38,10 +38,10 @@ describe("plugin API", function() {
   });
 
   describe("use", function() {
-    var grammar = 'start = "a"';
+    let grammar = 'start = "a"';
 
     it("is called for each plugin", function() {
-      var pluginsUsed = [false, false, false],
+      let pluginsUsed = [false, false, false],
           plugins     = [
             { use: function() { pluginsUsed[0] = true; } },
             { use: function() { pluginsUsed[1] = true; } },
@@ -54,7 +54,7 @@ describe("plugin API", function() {
     });
 
     it("receives configuration", function() {
-      var plugin = {
+      let plugin = {
             use: function(config) {
               expect(config).toBeObject();
 
@@ -84,7 +84,7 @@ describe("plugin API", function() {
     });
 
     it("receives options", function() {
-      var plugin             = {
+      let plugin             = {
             use: function(config, options) {
               expect(options).toEqual(generateOptions);
             }
@@ -95,9 +95,9 @@ describe("plugin API", function() {
     });
 
     it("can replace parser", function() {
-      var plugin = {
+      let plugin = {
             use: function(config) {
-              var parser = peg.generate([
+              let parser = peg.generate([
                     'start = .* {',
                     '  return {',
                     '    type:  "grammar",',
@@ -121,9 +121,9 @@ describe("plugin API", function() {
     });
 
     it("can change compiler passes", function() {
-      var plugin = {
+      let plugin = {
             use: function(config) {
-              var pass = function(ast) {
+              let pass = function(ast) {
                     ast.code = '({ parse: function() { return 42; } })';
                   };
 
@@ -136,7 +136,7 @@ describe("plugin API", function() {
     });
 
     it("can change options", function() {
-      var grammar = [
+      let grammar = [
             'a = "x"',
             'b = "x"',
             'c = "x"'
diff --git a/spec/behavior/generated-parser-behavior.spec.js b/spec/behavior/generated-parser-behavior.spec.js
index 2824569..2f54f8a 100644
--- a/spec/behavior/generated-parser-behavior.spec.js
+++ b/spec/behavior/generated-parser-behavior.spec.js
@@ -3,12 +3,12 @@
 
 "use strict";
 
-var peg = require("../../lib/peg");
+let peg = require("../../lib/peg");
 
 describe("generated parser behavior", function() {
   function varyOptimizationOptions(block) {
     function clone(object) {
-      var result = {}, key;
+      let result = {}, key;
 
       for (key in object) {
         if (object.hasOwnProperty(key)) {
@@ -19,7 +19,7 @@ describe("generated parser behavior", function() {
       return result;
     }
 
-    var optionsVariants = [
+    let optionsVariants = [
           { cache: false, optimize: "speed", trace: false },
           { cache: false, optimize: "speed", trace: true  },
           { cache: false, optimize: "size",  trace: false },
@@ -43,7 +43,7 @@ describe("generated parser behavior", function() {
       toParse: function(input, expected, options) {
         options = options !== undefined ? options : {};
 
-        var result;
+        let result;
 
         try {
           result = this.actual.parse(input, options);
@@ -77,7 +77,7 @@ describe("generated parser behavior", function() {
       toFailToParse: function(input, details, options) {
         options = options !== undefined ? options : {};
 
-        var result, key;
+        let result, key;
 
         try {
           result = this.actual.parse(input, options);
@@ -138,7 +138,7 @@ describe("generated parser behavior", function() {
   varyOptimizationOptions(function(options) {
     describe("initializer", function() {
       it("executes the code before parsing starts", function() {
-        var parser = peg.generate([
+        let parser = peg.generate([
               '{ var result = 42; }',
               'start = "a" { return result; }'
             ].join("\n"), options);
@@ -148,7 +148,7 @@ describe("generated parser behavior", function() {
 
       describe("available variables and functions", function() {
         it("|options| contains options", function() {
-          var parser = peg.generate([
+          let parser = peg.generate([
                 '{ var result = options; }',
                 'start = "a" { return result; }'
               ].join("\n"), options);
@@ -161,7 +161,7 @@ describe("generated parser behavior", function() {
     describe("rule", function() {
       if (options.cache) {
         it("caches rule match results", function() {
-          var parser = peg.generate([
+          let parser = peg.generate([
                 '{ var n = 0; }',
                 'start = (a "b") / (a "c") { return n; }',
                 'a = "a" { n++; }'
@@ -171,7 +171,7 @@ describe("generated parser behavior", function() {
         });
       } else {
         it("doesn't cache rule match results", function() {
-          var parser = peg.generate([
+          let parser = peg.generate([
                 '{ var n = 0; }',
                 'start = (a "b") / (a "c") { return n; }',
                 'a = "a" { n++; }'
@@ -183,7 +183,7 @@ describe("generated parser behavior", function() {
 
       describe("when the expression matches", function() {
         it("returns its match result", function() {
-          var parser = peg.generate('start = "a"');
+          let parser = peg.generate('start = "a"');
 
           expect(parser).toParse("a", "a");
         });
@@ -192,7 +192,7 @@ describe("generated parser behavior", function() {
       describe("when the expression doesn't match", function() {
         describe("without display name", function() {
           it("reports match failure and doesn't record any expectation", function() {
-            var parser = peg.generate('start = "a"');
+            let parser = peg.generate('start = "a"');
 
             expect(parser).toFailToParse("b", {
               expected: [{ type: "literal", text: "a", ignoreCase: false }]
@@ -202,7 +202,7 @@ describe("generated parser behavior", function() {
 
         describe("with display name", function() {
           it("reports match failure and records an expectation of type \"other\"", function() {
-            var parser = peg.generate('start "start" = "a"');
+            let parser = peg.generate('start "start" = "a"');
 
             expect(parser).toFailToParse("b", {
               expected: [{ type: "other", description: "start" }]
@@ -210,7 +210,7 @@ describe("generated parser behavior", function() {
           });
 
           it("discards any expectations recorded when matching the expression", function() {
-            var parser = peg.generate('start "start" = "a"');
+            let parser = peg.generate('start "start" = "a"');
 
             expect(parser).toFailToParse("b", {
               expected: [{ type: "other", description: "start" }]
@@ -223,34 +223,34 @@ describe("generated parser behavior", function() {
     describe("literal", function() {
       describe("matching", function() {
         it("matches empty literals", function() {
-          var parser = peg.generate('start = ""', options);
+          let parser = peg.generate('start = ""', options);
 
           expect(parser).toParse("");
         });
 
         it("matches one-character literals", function() {
-          var parser = peg.generate('start = "a"', options);
+          let parser = peg.generate('start = "a"', options);
 
           expect(parser).toParse("a");
           expect(parser).toFailToParse("b");
         });
 
         it("matches multi-character literals", function() {
-          var parser = peg.generate('start = "abcd"', options);
+          let parser = peg.generate('start = "abcd"', options);
 
           expect(parser).toParse("abcd");
           expect(parser).toFailToParse("efgh");
         });
 
         it("is case sensitive without the \"i\" flag", function() {
-          var parser = peg.generate('start = "a"', options);
+          let parser = peg.generate('start = "a"', options);
 
           expect(parser).toParse("a");
           expect(parser).toFailToParse("A");
         });
 
         it("is case insensitive with the \"i\" flag", function() {
-          var parser = peg.generate('start = "a"i', options);
+          let parser = peg.generate('start = "a"i', options);
 
           expect(parser).toParse("a");
           expect(parser).toParse("A");
@@ -259,13 +259,13 @@ describe("generated parser behavior", function() {
 
       describe("when it matches", function() {
         it("returns the matched text", function() {
-          var parser = peg.generate('start = "a"', options);
+          let parser = peg.generate('start = "a"', options);
 
           expect(parser).toParse("a", "a");
         });
 
         it("consumes the matched text", function() {
-          var parser = peg.generate('start = "a" .', options);
+          let parser = peg.generate('start = "a" .', options);
 
           expect(parser).toParse("ab");
         });
@@ -273,7 +273,7 @@ describe("generated parser behavior", function() {
 
       describe("when it doesn't match", function() {
         it("reports match failure and records an expectation of type \"literal\"", function() {
-          var parser = peg.generate('start = "a"', options);
+          let parser = peg.generate('start = "a"', options);
 
           expect(parser).toFailToParse("b", {
             expected: [{ type: "literal", text: "a", ignoreCase: false }]
@@ -285,13 +285,13 @@ describe("generated parser behavior", function() {
     describe("character class", function() {
       describe("matching", function() {
         it("matches empty classes", function() {
-          var parser = peg.generate('start = []', options);
+          let parser = peg.generate('start = []', options);
 
           expect(parser).toFailToParse("a");
         });
 
         it("matches classes with a character list", function() {
-          var parser = peg.generate('start = [abc]', options);
+          let parser = peg.generate('start = [abc]', options);
 
           expect(parser).toParse("a");
           expect(parser).toParse("b");
@@ -300,7 +300,7 @@ describe("generated parser behavior", function() {
         });
 
         it("matches classes with a character range", function() {
-          var parser = peg.generate('start = [a-c]', options);
+          let parser = peg.generate('start = [a-c]', options);
 
           expect(parser).toParse("a");
           expect(parser).toParse("b");
@@ -309,21 +309,21 @@ describe("generated parser behavior", function() {
         });
 
         it("matches inverted classes", function() {
-          var parser = peg.generate('start = [^a]', options);
+          let parser = peg.generate('start = [^a]', options);
 
           expect(parser).toFailToParse("a");
           expect(parser).toParse("b");
         });
 
         it("is case sensitive without the \"i\" flag", function() {
-          var parser = peg.generate('start = [a]', options);
+          let parser = peg.generate('start = [a]', options);
 
           expect(parser).toParse("a");
           expect(parser).toFailToParse("A");
         });
 
         it("is case insensitive with the \"i\" flag", function() {
-          var parser = peg.generate('start = [a]i', options);
+          let parser = peg.generate('start = [a]i', options);
 
           expect(parser).toParse("a");
           expect(parser).toParse("A");
@@ -332,13 +332,13 @@ describe("generated parser behavior", function() {
 
       describe("when it matches", function() {
         it("returns the matched character", function() {
-          var parser = peg.generate('start = [a]', options);
+          let parser = peg.generate('start = [a]', options);
 
           expect(parser).toParse("a", "a");
         });
 
         it("consumes the matched character", function() {
-          var parser = peg.generate('start = [a] .', options);
+          let parser = peg.generate('start = [a] .', options);
 
           expect(parser).toParse("ab");
         });
@@ -346,7 +346,7 @@ describe("generated parser behavior", function() {
 
       describe("when it doesn't match", function() {
         it("reports match failure and records an expectation of type \"class\"", function() {
-          var parser = peg.generate('start = [a]', options);
+          let parser = peg.generate('start = [a]', options);
 
           expect(parser).toFailToParse("b", {
             expected: [{ type: "class", parts: ["a"], inverted: false, ignoreCase: false }]
@@ -358,7 +358,7 @@ describe("generated parser behavior", function() {
     describe("dot", function() {
       describe("matching", function() {
         it("matches any character", function() {
-          var parser = peg.generate('start = .', options);
+          let parser = peg.generate('start = .', options);
 
           expect(parser).toParse("a");
           expect(parser).toParse("b");
@@ -368,13 +368,13 @@ describe("generated parser behavior", function() {
 
       describe("when it matches", function() {
         it("returns the matched character", function() {
-          var parser = peg.generate('start = .', options);
+          let parser = peg.generate('start = .', options);
 
           expect(parser).toParse("a", "a");
         });
 
         it("consumes the matched character", function() {
-          var parser = peg.generate('start = . .', options);
+          let parser = peg.generate('start = . .', options);
 
           expect(parser).toParse("ab");
         });
@@ -382,7 +382,7 @@ describe("generated parser behavior", function() {
 
       describe("when it doesn't match", function() {
         it("reports match failure and records an expectation of type \"any\"", function() {
-          var parser = peg.generate('start = .', options);
+          let parser = peg.generate('start = .', options);
 
           expect(parser).toFailToParse("", {
             expected: [{ type: "any" }]
@@ -394,7 +394,7 @@ describe("generated parser behavior", function() {
     describe("rule reference", function() {
       describe("when referenced rule's expression matches", function() {
         it("returns its result", function() {
-          var parser = peg.generate([
+          let parser = peg.generate([
                 'start = a',
                 'a = "a"'
               ].join("\n"), options);
@@ -405,7 +405,7 @@ describe("generated parser behavior", function() {
 
       describe("when referenced rule's expression doesn't match", function() {
         it("reports match failure", function() {
-          var parser = peg.generate([
+          let parser = peg.generate([
                 'start = a',
                 'a = "a"'
               ].join("\n"), options);
@@ -423,7 +423,7 @@ describe("generated parser behavior", function() {
            * |undefined| which we can't compare against in |toParse| due to the
            * way optional parameters work.
            */
-          var parser = peg.generate('start = &{ return true; } ""', options);
+          let parser = peg.generate('start = &{ return true; } ""', options);
 
           expect(parser).toParse("", [undefined, ""]);
         });
@@ -431,7 +431,7 @@ describe("generated parser behavior", function() {
 
       describe("when the code returns a falsey value", function() {
         it("reports match failure", function() {
-          var parser = peg.generate('start = &{ return false; }', options);
+          let parser = peg.generate('start = &{ return false; }', options);
 
           expect(parser).toFailToParse("");
         });
@@ -440,7 +440,7 @@ describe("generated parser behavior", function() {
       describe("label variables", function() {
         describe("in containing sequence", function() {
           it("can access variables defined by preceding labeled elements", function() {
-            var parser = peg.generate(
+            let parser = peg.generate(
                   'start = a:"a" &{ return a === "a"; }',
                   options
                 );
@@ -449,7 +449,7 @@ describe("generated parser behavior", function() {
           });
 
           it("cannot access variable defined by labeled predicate element", function() {
-            var parser = peg.generate(
+            let parser = peg.generate(
                   'start = "a" b:&{ return b === undefined; } "c"',
                   options
                 );
@@ -458,7 +458,7 @@ describe("generated parser behavior", function() {
           });
 
           it("cannot access variables defined by following labeled elements", function() {
-            var parser = peg.generate(
+            let parser = peg.generate(
                   'start = &{ return a === "a"; } a:"a"',
                   options
                 );
@@ -467,7 +467,7 @@ describe("generated parser behavior", function() {
           });
 
           it("cannot access variables defined by subexpressions", function() {
-            var testcases = [
+            let testcases = [
                   {
                     grammar: 'start = (a:"a") &{ return a === "a"; }',
                     input:   "a"
@@ -524,7 +524,7 @@ describe("generated parser behavior", function() {
 
         describe("in outer sequence", function() {
           it("can access variables defined by preceding labeled elements", function() {
-            var parser = peg.generate(
+            let parser = peg.generate(
                   'start = a:"a" ("b" &{ return a === "a"; })',
                   options
                 );
@@ -533,7 +533,7 @@ describe("generated parser behavior", function() {
           });
 
           it("cannot access variable defined by labeled predicate element", function() {
-            var parser = peg.generate(
+            let parser = peg.generate(
                   'start = "a" b:("b" &{ return b === undefined; }) "c"',
                   options
                 );
@@ -542,7 +542,7 @@ describe("generated parser behavior", function() {
           });
 
           it("cannot access variables defined by following labeled elements", function() {
-            var parser = peg.generate(
+            let parser = peg.generate(
                   'start = ("a" &{ return b === "b"; }) b:"b"',
                   options
                 );
@@ -554,7 +554,7 @@ describe("generated parser behavior", function() {
 
       describe("initializer variables & functions", function() {
         it("can access variables defined in the initializer", function() {
-          var parser = peg.generate([
+          let parser = peg.generate([
                 '{ var v = 42 }',
                 'start = &{ return v === 42; }'
               ].join("\n"), options);
@@ -563,7 +563,7 @@ describe("generated parser behavior", function() {
         });
 
         it("can access functions defined in the initializer", function() {
-          var parser = peg.generate([
+          let parser = peg.generate([
                 '{ function f() { return 42; } }',
                 'start = &{ return f() === 42; }'
               ].join("\n"), options);
@@ -574,7 +574,7 @@ describe("generated parser behavior", function() {
 
       describe("available variables & functions", function() {
         it("|options| contains options", function() {
-          var parser = peg.generate([
+          let parser = peg.generate([
                 '{ var result; }',
                 'start = &{ result = options; return true; } { return result; }'
               ].join("\n"), options);
@@ -583,7 +583,7 @@ describe("generated parser behavior", function() {
         });
 
         it("|location| returns current location info", function() {
-          var parser = peg.generate([
+          let parser = peg.generate([
                 '{ var result; }',
                 'start  = line (nl+ line)* { return result; }',
                 'line   = thing (" "+ thing)*',
@@ -619,7 +619,7 @@ describe("generated parser behavior", function() {
            * |undefined| which we can't compare against in |toParse| due to the
            * way optional parameters work.
            */
-          var parser = peg.generate('start = !{ return false; } ""', options);
+          let parser = peg.generate('start = !{ return false; } ""', options);
 
           expect(parser).toParse("", [undefined, ""]);
         });
@@ -627,7 +627,7 @@ describe("generated parser behavior", function() {
 
       describe("when the code returns a truthy value", function() {
         it("reports match failure", function() {
-          var parser = peg.generate('start = !{ return true; }', options);
+          let parser = peg.generate('start = !{ return true; }', options);
 
           expect(parser).toFailToParse("");
         });
@@ -636,7 +636,7 @@ describe("generated parser behavior", function() {
       describe("label variables", function() {
         describe("in containing sequence", function() {
           it("can access variables defined by preceding labeled elements", function() {
-            var parser = peg.generate(
+            let parser = peg.generate(
                   'start = a:"a" !{ return a !== "a"; }',
                   options
                 );
@@ -645,7 +645,7 @@ describe("generated parser behavior", function() {
           });
 
           it("cannot access variable defined by labeled predicate element", function() {
-            var parser = peg.generate(
+            let parser = peg.generate(
                   'start = "a" b:!{ return b !== undefined; } "c"',
                   options
                 );
@@ -654,7 +654,7 @@ describe("generated parser behavior", function() {
           });
 
           it("cannot access variables defined by following labeled elements", function() {
-            var parser = peg.generate(
+            let parser = peg.generate(
                   'start = !{ return a !== "a"; } a:"a"',
                   options
                 );
@@ -663,7 +663,7 @@ describe("generated parser behavior", function() {
           });
 
           it("cannot access variables defined by subexpressions", function() {
-            var testcases = [
+            let testcases = [
                   {
                     grammar: 'start = (a:"a") !{ return a !== "a"; }',
                     input:   "a"
@@ -720,7 +720,7 @@ describe("generated parser behavior", function() {
 
         describe("in outer sequence", function() {
           it("can access variables defined by preceding labeled elements", function() {
-            var parser = peg.generate(
+            let parser = peg.generate(
                   'start = a:"a" ("b" !{ return a !== "a"; })',
                   options
                 );
@@ -729,7 +729,7 @@ describe("generated parser behavior", function() {
           });
 
           it("cannot access variable defined by labeled predicate element", function() {
-            var parser = peg.generate(
+            let parser = peg.generate(
                   'start = "a" b:("b" !{ return b !== undefined; }) "c"',
                   options
                 );
@@ -738,7 +738,7 @@ describe("generated parser behavior", function() {
           });
 
           it("cannot access variables defined by following labeled elements", function() {
-            var parser = peg.generate(
+            let parser = peg.generate(
                   'start = ("a" !{ return b !== "b"; }) b:"b"',
                   options
                 );
@@ -750,7 +750,7 @@ describe("generated parser behavior", function() {
 
       describe("initializer variables & functions", function() {
         it("can access variables defined in the initializer", function() {
-          var parser = peg.generate([
+          let parser = peg.generate([
                 '{ var v = 42 }',
                 'start = !{ return v !== 42; }'
               ].join("\n"), options);
@@ -759,7 +759,7 @@ describe("generated parser behavior", function() {
         });
 
         it("can access functions defined in the initializer", function() {
-          var parser = peg.generate([
+          let parser = peg.generate([
                 '{ function f() { return 42; } }',
                 'start = !{ return f() !== 42; }'
               ].join("\n"), options);
@@ -770,7 +770,7 @@ describe("generated parser behavior", function() {
 
       describe("available variables & functions", function() {
         it("|options| contains options", function() {
-          var parser = peg.generate([
+          let parser = peg.generate([
                 '{ var result; }',
                 'start = !{ result = options; return false; } { return result; }'
               ].join("\n"), options);
@@ -779,7 +779,7 @@ describe("generated parser behavior", function() {
         });
 
         it("|location| returns current location info", function() {
-          var parser = peg.generate([
+          let parser = peg.generate([
                 '{ var result; }',
                 'start  = line (nl+ line)* { return result; }',
                 'line   = thing (" "+ thing)*',
@@ -810,7 +810,7 @@ describe("generated parser behavior", function() {
     describe("group", function() {
       describe("when the expression matches", function() {
         it("returns its match result", function() {
-          var parser = peg.generate('start = ("a")', options);
+          let parser = peg.generate('start = ("a")', options);
 
           expect(parser).toParse("a", "a");
         });
@@ -818,7 +818,7 @@ describe("generated parser behavior", function() {
 
       describe("when the expression doesn't match", function() {
         it("reports match failure", function() {
-          var parser = peg.generate('start = ("a")', options);
+          let parser = peg.generate('start = ("a")', options);
 
           expect(parser).toFailToParse("b");
         });
@@ -828,7 +828,7 @@ describe("generated parser behavior", function() {
     describe("optional", function() {
       describe("when the expression matches", function() {
         it("returns its match result", function() {
-          var parser = peg.generate('start = "a"?', options);
+          let parser = peg.generate('start = "a"?', options);
 
           expect(parser).toParse("a", "a");
         });
@@ -836,7 +836,7 @@ describe("generated parser behavior", function() {
 
       describe("when the expression doesn't match", function() {
         it("returns |null|", function() {
-          var parser = peg.generate('start = "a"?', options);
+          let parser = peg.generate('start = "a"?', options);
 
           expect(parser).toParse("", null);
         });
@@ -846,7 +846,7 @@ describe("generated parser behavior", function() {
     describe("zero or more", function() {
       describe("when the expression matches zero or more times", function() {
         it("returns an array of its match results", function() {
-          var parser = peg.generate('start = "a"*', options);
+          let parser = peg.generate('start = "a"*', options);
 
           expect(parser).toParse("",    []);
           expect(parser).toParse("a",   ["a"]);
@@ -858,7 +858,7 @@ describe("generated parser behavior", function() {
     describe("one or more", function() {
       describe("when the expression matches one or more times", function() {
         it("returns an array of its match results", function() {
-          var parser = peg.generate('start = "a"+', options);
+          let parser = peg.generate('start = "a"+', options);
 
           expect(parser).toParse("a",   ["a"]);
           expect(parser).toParse("aaa", ["a", "a", "a"]);
@@ -867,7 +867,7 @@ describe("generated parser behavior", function() {
 
       describe("when the expression doesn't match", function() {
         it("reports match failure", function() {
-          var parser = peg.generate('start = "a"+', options);
+          let parser = peg.generate('start = "a"+', options);
 
           expect(parser).toFailToParse("");
         });
@@ -877,7 +877,7 @@ describe("generated parser behavior", function() {
     describe("text", function() {
       describe("when the expression matches", function() {
         it("returns the matched text", function() {
-          var parser = peg.generate('start = $("a" "b" "c")', options);
+          let parser = peg.generate('start = $("a" "b" "c")', options);
 
           expect(parser).toParse("abc", "abc");
         });
@@ -885,7 +885,7 @@ describe("generated parser behavior", function() {
 
       describe("when the expression doesn't match", function() {
         it("reports match failure", function() {
-          var parser = peg.generate('start = $("a")', options);
+          let parser = peg.generate('start = $("a")', options);
 
           expect(parser).toFailToParse("b");
         });
@@ -895,13 +895,13 @@ describe("generated parser behavior", function() {
     describe("positive simple predicate", function() {
       describe("when the expression matches", function() {
         it("returns |undefined|", function() {
-          var parser = peg.generate('start = &"a" "a"', options);
+          let parser = peg.generate('start = &"a" "a"', options);
 
           expect(parser).toParse("a", [undefined, "a"]);
         });
 
         it("resets parse position", function() {
-          var parser = peg.generate('start = &"a" "a"', options);
+          let parser = peg.generate('start = &"a" "a"', options);
 
           expect(parser).toParse("a");
         });
@@ -909,13 +909,13 @@ describe("generated parser behavior", function() {
 
       describe("when the expression doesn't match", function() {
         it("reports match failure", function() {
-          var parser = peg.generate('start = &"a"', options);
+          let parser = peg.generate('start = &"a"', options);
 
           expect(parser).toFailToParse("b");
         });
 
         it("discards any expectations recorded when matching the expression", function() {
-          var parser = peg.generate('start = "a" / &"b" / "c"', options);
+          let parser = peg.generate('start = "a" / &"b" / "c"', options);
 
           expect(parser).toFailToParse("d", {
             expected: [
@@ -930,7 +930,7 @@ describe("generated parser behavior", function() {
     describe("negative simple predicate", function() {
       describe("when the expression matches", function() {
         it("reports match failure", function() {
-          var parser = peg.generate('start = !"a"', options);
+          let parser = peg.generate('start = !"a"', options);
 
           expect(parser).toFailToParse("a");
         });
@@ -938,19 +938,19 @@ describe("generated parser behavior", function() {
 
       describe("when the expression doesn't match", function() {
         it("returns |undefined|", function() {
-          var parser = peg.generate('start = !"a" "b"', options);
+          let parser = peg.generate('start = !"a" "b"', options);
 
           expect(parser).toParse("b", [undefined, "b"]);
         });
 
         it("resets parse position", function() {
-          var parser = peg.generate('start = !"a" "b"', options);
+          let parser = peg.generate('start = !"a" "b"', options);
 
           expect(parser).toParse("b");
         });
 
         it("discards any expectations recorded when matching the expression", function() {
-          var parser = peg.generate('start = "a" / !"b" / "c"', options);
+          let parser = peg.generate('start = "a" / !"b" / "c"', options);
 
           expect(parser).toFailToParse("b", {
             expected: [
@@ -965,7 +965,7 @@ describe("generated parser behavior", function() {
     describe("label", function() {
       describe("when the expression matches", function() {
         it("returns its match result", function() {
-          var parser = peg.generate('start = a:"a"', options);
+          let parser = peg.generate('start = a:"a"', options);
 
           expect(parser).toParse("a", "a");
         });
@@ -973,7 +973,7 @@ describe("generated parser behavior", function() {
 
       describe("when the expression doesn't match", function() {
         it("reports match failure", function() {
-          var parser = peg.generate('start = a:"a"', options);
+          let parser = peg.generate('start = a:"a"', options);
 
           expect(parser).toFailToParse("b");
         });
@@ -983,7 +983,7 @@ describe("generated parser behavior", function() {
     describe("sequence", function() {
       describe("when all expressions match", function() {
         it("returns an array of their match results", function() {
-          var parser = peg.generate('start = "a" "b" "c"', options);
+          let parser = peg.generate('start = "a" "b" "c"', options);
 
           expect(parser).toParse("abc", ["a", "b", "c"]);
         });
@@ -991,7 +991,7 @@ describe("generated parser behavior", function() {
 
       describe("when any expression doesn't match", function() {
         it("reports match failure", function() {
-          var parser = peg.generate('start = "a" "b" "c"', options);
+          let parser = peg.generate('start = "a" "b" "c"', options);
 
           expect(parser).toFailToParse("dbc");
           expect(parser).toFailToParse("adc");
@@ -999,7 +999,7 @@ describe("generated parser behavior", function() {
         });
 
         it("resets parse position", function() {
-          var parser = peg.generate('start = "a" "b" / "a"', options);
+          let parser = peg.generate('start = "a" "b" / "a"', options);
 
           expect(parser).toParse("a", "a");
         });
@@ -1009,7 +1009,7 @@ describe("generated parser behavior", function() {
     describe("action", function() {
       describe("when the expression matches", function() {
         it("returns the value returned by the code", function() {
-          var parser = peg.generate('start = "a" { return 42; }', options);
+          let parser = peg.generate('start = "a" { return 42; }', options);
 
           expect(parser).toParse("a", 42);
         });
@@ -1017,13 +1017,13 @@ describe("generated parser behavior", function() {
         describe("label variables", function() {
           describe("in the expression", function() {
             it("can access variable defined by labeled expression", function() {
-              var parser = peg.generate('start = a:"a" { return a; }', options);
+              let parser = peg.generate('start = a:"a" { return a; }', options);
 
               expect(parser).toParse("a", "a");
             });
 
             it("can access variables defined by labeled sequence elements", function() {
-              var parser = peg.generate(
+              let parser = peg.generate(
                     'start = a:"a" b:"b" c:"c" { return [a, b, c]; }',
                     options
                   );
@@ -1032,7 +1032,7 @@ describe("generated parser behavior", function() {
             });
 
             it("cannot access variables defined by subexpressions", function() {
-              var testcases = [
+              let testcases = [
                     {
                       grammar: 'start = (a:"a") { return a; }',
                       input:   "a"
@@ -1089,7 +1089,7 @@ describe("generated parser behavior", function() {
 
           describe("in outer sequence", function() {
             it("can access variables defined by preceding labeled elements", function() {
-              var parser = peg.generate(
+              let parser = peg.generate(
                     'start = a:"a" ("b" { return a; })',
                     options
                   );
@@ -1098,7 +1098,7 @@ describe("generated parser behavior", function() {
             });
 
             it("cannot access variable defined by labeled action element", function() {
-              var parser = peg.generate(
+              let parser = peg.generate(
                     'start = "a" b:("b" { return b; }) c:"c"',
                     options
                   );
@@ -1107,7 +1107,7 @@ describe("generated parser behavior", function() {
             });
 
             it("cannot access variables defined by following labeled elements", function() {
-              var parser = peg.generate(
+              let parser = peg.generate(
                     'start = ("a" { return b; }) b:"b"',
                     options
                   );
@@ -1119,7 +1119,7 @@ describe("generated parser behavior", function() {
 
         describe("initializer variables & functions", function() {
           it("can access variables defined in the initializer", function() {
-            var parser = peg.generate([
+            let parser = peg.generate([
                   '{ var v = 42 }',
                   'start = "a" { return v; }'
                 ].join("\n"), options);
@@ -1128,7 +1128,7 @@ describe("generated parser behavior", function() {
           });
 
           it("can access functions defined in the initializer", function() {
-            var parser = peg.generate([
+            let parser = peg.generate([
                   '{ function f() { return 42; } }',
                   'start = "a" { return f(); }'
                 ].join("\n"), options);
@@ -1139,7 +1139,7 @@ describe("generated parser behavior", function() {
 
         describe("available variables & functions", function() {
           it("|options| contains options", function() {
-            var parser = peg.generate(
+            let parser = peg.generate(
                   'start = "a" { return options; }',
                   options
                 );
@@ -1148,7 +1148,7 @@ describe("generated parser behavior", function() {
           });
 
           it("|text| returns text matched by the expression", function() {
-            var parser = peg.generate(
+            let parser = peg.generate(
                   'start = "a" { return text(); }',
                   options
                 );
@@ -1157,7 +1157,7 @@ describe("generated parser behavior", function() {
           });
 
           it("|location| returns location info of the expression", function() {
-            var parser = peg.generate([
+            let parser = peg.generate([
                   '{ var result; }',
                   'start  = line (nl+ line)* { return result; }',
                   'line   = thing (" "+ thing)*',
@@ -1185,7 +1185,7 @@ describe("generated parser behavior", function() {
 
           describe("|expected|", function() {
             it("terminates parsing and throws an exception", function() {
-              var parser = peg.generate(
+              let parser = peg.generate(
                     'start = "a" { expected("a"); }',
                     options
                   );
@@ -1202,7 +1202,7 @@ describe("generated parser behavior", function() {
             });
 
             it("allows to set custom location info", function() {
-              var parser = peg.generate([
+              let parser = peg.generate([
                     'start = "a" {',
                     '  expected("a", {',
                     '    start: { offset: 1, line: 1, column: 2 },',
@@ -1225,7 +1225,7 @@ describe("generated parser behavior", function() {
 
           describe("|error|", function() {
             it("terminates parsing and throws an exception", function() {
-              var parser = peg.generate(
+              let parser = peg.generate(
                     'start = "a" { error("a"); }',
                     options
                   );
@@ -1242,7 +1242,7 @@ describe("generated parser behavior", function() {
             });
 
             it("allows to set custom location info", function() {
-              var parser = peg.generate([
+              let parser = peg.generate([
                     'start = "a" {',
                     '  error("a", {',
                     '    start: { offset: 1, line: 1, column: 2 },',
@@ -1267,13 +1267,13 @@ describe("generated parser behavior", function() {
 
       describe("when the expression doesn't match", function() {
         it("reports match failure", function() {
-          var parser = peg.generate('start = "a" { return 42; }', options);
+          let parser = peg.generate('start = "a" { return 42; }', options);
 
           expect(parser).toFailToParse("b");
         });
 
         it("doesn't execute the code", function() {
-          var parser = peg.generate(
+          let parser = peg.generate(
                 'start = "a" { throw "Boom!"; } / "b"',
                 options
               );
@@ -1286,7 +1286,7 @@ describe("generated parser behavior", function() {
     describe("choice", function() {
       describe("when any expression matches", function() {
         it("returns its match result", function() {
-          var parser = peg.generate('start = "a" / "b" / "c"', options);
+          let parser = peg.generate('start = "a" / "b" / "c"', options);
 
           expect(parser).toParse("a", "a");
           expect(parser).toParse("b", "b");
@@ -1296,7 +1296,7 @@ describe("generated parser behavior", function() {
 
       describe("when all expressions don't match", function() {
         it("reports match failure", function() {
-          var parser = peg.generate('start = "a" / "b" / "c"', options);
+          let parser = peg.generate('start = "a" / "b" / "c"', options);
 
           expect(parser).toFailToParse("d");
         });
@@ -1306,7 +1306,7 @@ describe("generated parser behavior", function() {
     describe("error reporting", function() {
       describe("behavior", function() {
         it("reports only the rightmost error", function() {
-          var parser = peg.generate('start = "a" "b" / "a" "c" "d"', options);
+          let parser = peg.generate('start = "a" "b" / "a" "c" "d"', options);
 
           expect(parser).toFailToParse("ace", {
             expected: [{ type: "literal", text: "d", ignoreCase: false }]
@@ -1316,7 +1316,7 @@ describe("generated parser behavior", function() {
 
       describe("expectations reporting", function() {
         it("reports expectations correctly with no alternative", function() {
-          var parser = peg.generate('start = "a"', options);
+          let parser = peg.generate('start = "a"', options);
 
           expect(parser).toFailToParse("ab", {
             expected: [{ type: "end" }]
@@ -1324,7 +1324,7 @@ describe("generated parser behavior", function() {
         });
 
         it("reports expectations correctly with one alternative", function() {
-          var parser = peg.generate('start = "a"', options);
+          let parser = peg.generate('start = "a"', options);
 
           expect(parser).toFailToParse("b", {
             expected: [{ type: "literal", text: "a", ignoreCase: false }]
@@ -1332,7 +1332,7 @@ describe("generated parser behavior", function() {
         });
 
         it("reports expectations correctly with multiple alternatives", function() {
-          var parser = peg.generate('start = "a" / "b" / "c"', options);
+          let parser = peg.generate('start = "a" / "b" / "c"', options);
 
           expect(parser).toFailToParse("d", {
             expected: [
@@ -1346,13 +1346,13 @@ describe("generated parser behavior", function() {
 
       describe("found string reporting", function() {
         it("reports found string correctly at the end of input", function() {
-          var parser = peg.generate('start = "a"', options);
+          let parser = peg.generate('start = "a"', options);
 
           expect(parser).toFailToParse("", { found: null });
         });
 
         it("reports found string correctly in the middle of input", function() {
-          var parser = peg.generate('start = "a"', options);
+          let parser = peg.generate('start = "a"', options);
 
           expect(parser).toFailToParse("b", { found: "b" });
         });
@@ -1360,7 +1360,7 @@ describe("generated parser behavior", function() {
 
       describe("message building", function() {
         it("builds message correctly with no alternative", function() {
-          var parser = peg.generate('start = "a"', options);
+          let parser = peg.generate('start = "a"', options);
 
           expect(parser).toFailToParse("ab", {
             message: 'Expected end of input but "b" found.'
@@ -1368,7 +1368,7 @@ describe("generated parser behavior", function() {
         });
 
         it("builds message correctly with one alternative", function() {
-          var parser = peg.generate('start = "a"', options);
+          let parser = peg.generate('start = "a"', options);
 
           expect(parser).toFailToParse("b", {
             message: 'Expected "a" but "b" found.'
@@ -1376,7 +1376,7 @@ describe("generated parser behavior", function() {
         });
 
         it("builds message correctly with multiple alternatives", function() {
-          var parser = peg.generate('start = "a" / "b" / "c"', options);
+          let parser = peg.generate('start = "a" / "b" / "c"', options);
 
           expect(parser).toFailToParse("d", {
             message: 'Expected "a", "b", or "c" but "d" found.'
@@ -1384,7 +1384,7 @@ describe("generated parser behavior", function() {
         });
 
         it("builds message correctly at the end of input", function() {
-          var parser = peg.generate('start = "a"', options);
+          let parser = peg.generate('start = "a"', options);
 
           expect(parser).toFailToParse("", {
             message: 'Expected "a" but end of input found.'
@@ -1392,7 +1392,7 @@ describe("generated parser behavior", function() {
         });
 
         it("builds message correctly in the middle of input", function() {
-          var parser = peg.generate('start = "a"', options);
+          let parser = peg.generate('start = "a"', options);
 
           expect(parser).toFailToParse("b", {
             message: 'Expected "a" but "b" found.'
@@ -1400,7 +1400,7 @@ describe("generated parser behavior", function() {
         });
 
         it("removes duplicates from expectations", function() {
-          var parser = peg.generate('start = "a" / "a"', options);
+          let parser = peg.generate('start = "a" / "a"', options);
 
           expect(parser).toFailToParse("b", {
             message: 'Expected "a" but "b" found.'
@@ -1408,7 +1408,7 @@ describe("generated parser behavior", function() {
         });
 
         it("sorts expectations", function() {
-          var parser = peg.generate('start = "c" / "b" / "a"', options);
+          let parser = peg.generate('start = "c" / "b" / "a"', options);
 
           expect(parser).toFailToParse("d", {
             message: 'Expected "a", "b", or "c" but "d" found.'
@@ -1419,7 +1419,7 @@ describe("generated parser behavior", function() {
 
       describe("position reporting", function() {
         it("reports position correctly at the end of input", function() {
-          var parser = peg.generate('start = "a"', options);
+          let parser = peg.generate('start = "a"', options);
 
           expect(parser).toFailToParse("", {
             location: {
@@ -1430,7 +1430,7 @@ describe("generated parser behavior", function() {
         });
 
         it("reports position correctly in the middle of input", function() {
-          var parser = peg.generate('start = "a"', options);
+          let parser = peg.generate('start = "a"', options);
 
           expect(parser).toFailToParse("b", {
             location: {
@@ -1441,7 +1441,7 @@ describe("generated parser behavior", function() {
         });
 
         it("reports position correctly with trailing input", function() {
-          var parser = peg.generate('start = "a"', options);
+          let parser = peg.generate('start = "a"', options);
 
           expect(parser).toFailToParse("aa", {
             location: {
@@ -1452,7 +1452,7 @@ describe("generated parser behavior", function() {
         });
 
         it("reports position correctly in complex cases", function() {
-          var parser = peg.generate([
+          let parser = peg.generate([
                 'start  = line (nl+ line)*',
                 'line   = digit (" "+ digit)*',
                 'digit  = [0-9]',
@@ -1495,7 +1495,7 @@ describe("generated parser behavior", function() {
          * Sum     ← Product (('+' / '-') Product)*
          * Expr    ← Sum
          */
-        var parser = peg.generate([
+        let parser = peg.generate([
               'Expr    = Sum',
               'Sum     = head:Product tail:(("+" / "-") Product)* {',
               '            return tail.reduce(function(result, element) {',
@@ -1547,7 +1547,7 @@ describe("generated parser behavior", function() {
          * A ← a A? b
          * B ← b B? c
          */
-        var parser = peg.generate([
+        let parser = peg.generate([
               'S = &(A "c") a:"a"+ B:B !("a" / "b" / "c") { return a.join("") + B; }',
               'A = a:"a" A:A? b:"b" { return [a, A, b].join(""); }',
               'B = b:"b" B:B? c:"c" { return [b, B, c].join(""); }'
@@ -1571,7 +1571,7 @@ describe("generated parser behavior", function() {
          * N ← C / (!Begin !End Z)
          * Z ← any single character
          */
-        var parser = peg.generate([
+        let parser = peg.generate([
               'C     = begin:Begin ns:N* end:End { return begin + ns.join("") + end; }',
               'N     = C',
               '      / !Begin !End z:Z { return z; }',
diff --git a/spec/server b/spec/server
index 5076728..d6e7526 100755
--- a/spec/server
+++ b/spec/server
@@ -9,19 +9,19 @@
  * browser.
  */
 
-var express    = require("express"),
+let express    = require("express"),
     logger     = require("morgan"),
     glob       = require("glob"),
     browserify = require("browserify"),
     babelify   = require("babelify");
 
-var app = express();
+let app = express();
 
 app.use(logger("dev"));
 app.use(express.static(__dirname));
 
 app.get("/bundle.js", function(req, res) {
-  var files = glob.sync(__dirname + "/**/*.js", {
+  let files = glob.sync(__dirname + "/**/*.js", {
     ignore: __dirname + "/vendor/**/*"
   });
 
diff --git a/spec/unit/compiler/passes/generate-bytecode.spec.js b/spec/unit/compiler/passes/generate-bytecode.spec.js
index 0071a7b..cb6334d 100644
--- a/spec/unit/compiler/passes/generate-bytecode.spec.js
+++ b/spec/unit/compiler/passes/generate-bytecode.spec.js
@@ -1,9 +1,9 @@
 "use strict";
 
-var peg = require("../../../../lib/peg");
+let peg = require("../../../../lib/peg");
 
 describe("compiler pass |generateBytecode|", function() {
-  var pass = peg.compiler.passes.generate.generateBytecode;
+  let pass = peg.compiler.passes.generate.generateBytecode;
 
   function bytecodeDetails(bytecode) {
     return {
@@ -53,7 +53,7 @@ describe("compiler pass |generateBytecode|", function() {
   });
 
   describe("for named", function() {
-    var grammar = 'start "start" = "a"';
+    let grammar = 'start "start" = "a"';
 
     it("generates correct bytecode", function() {
       expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -90,7 +90,7 @@ describe("compiler pass |generateBytecode|", function() {
 
   describe("for action", function() {
     describe("without labels", function() {
-      var grammar = 'start = "a" { code }';
+      let grammar = 'start = "a" { code }';
 
       it("generates correct bytecode", function() {
         expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -113,7 +113,7 @@ describe("compiler pass |generateBytecode|", function() {
     });
 
     describe("with one label", function() {
-      var grammar = 'start = a:"a" { code }';
+      let grammar = 'start = a:"a" { code }';
 
       it("generates correct bytecode", function() {
         expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -136,7 +136,7 @@ describe("compiler pass |generateBytecode|", function() {
     });
 
     describe("with multiple labels", function() {
-      var grammar = 'start = a:"a" b:"b" c:"c" { code }';
+      let grammar = 'start = a:"a" b:"b" c:"c" { code }';
 
       it("generates correct bytecode", function() {
         expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -177,7 +177,7 @@ describe("compiler pass |generateBytecode|", function() {
   });
 
   describe("for sequence", function() {
-    var grammar = 'start = "a" "b" "c"';
+    let grammar = 'start = "a" "b" "c"';
 
     it("generates correct bytecode", function() {
       expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -236,7 +236,7 @@ describe("compiler pass |generateBytecode|", function() {
   });
 
   describe("for simple_and", function() {
-    var grammar = 'start = &"a"';
+    let grammar = 'start = &"a"';
 
     it("generates correct bytecode", function() {
       expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -263,7 +263,7 @@ describe("compiler pass |generateBytecode|", function() {
   });
 
   describe("for simple_not", function() {
-    var grammar = 'start = !"a"';
+    let grammar = 'start = !"a"';
 
     it("generates correct bytecode", function() {
       expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -290,7 +290,7 @@ describe("compiler pass |generateBytecode|", function() {
   });
 
   describe("for optional", function() {
-    var grammar = 'start = "a"?';
+    let grammar = 'start = "a"?';
 
     it("generates correct bytecode", function() {
       expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -310,7 +310,7 @@ describe("compiler pass |generateBytecode|", function() {
   });
 
   describe("for zero_or_more", function() {
-    var grammar = 'start = "a"*';
+    let grammar = 'start = "a"*';
 
     it("generates correct bytecode", function() {
       expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -332,7 +332,7 @@ describe("compiler pass |generateBytecode|", function() {
   });
 
   describe("for one_or_more", function() {
-    var grammar = 'start = "a"+';
+    let grammar = 'start = "a"+';
 
     it("generates correct bytecode", function() {
       expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -367,7 +367,7 @@ describe("compiler pass |generateBytecode|", function() {
 
   describe("for semantic_and", function() {
     describe("without labels", function() {
-      var grammar = 'start = &{ code }';
+      let grammar = 'start = &{ code }';
 
       it("generates correct bytecode", function() {
         expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -390,7 +390,7 @@ describe("compiler pass |generateBytecode|", function() {
     });
 
     describe("with labels", function() {
-      var grammar = 'start = a:"a" b:"b" c:"c" &{ code }';
+      let grammar = 'start = a:"a" b:"b" c:"c" &{ code }';
 
       it("generates correct bytecode", function() {
         expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -442,7 +442,7 @@ describe("compiler pass |generateBytecode|", function() {
 
   describe("for semantic_not", function() {
     describe("without labels", function() {
-      var grammar = 'start = !{ code }';
+      let grammar = 'start = !{ code }';
 
       it("generates correct bytecode", function() {
         expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -465,7 +465,7 @@ describe("compiler pass |generateBytecode|", function() {
     });
 
     describe("with labels", function() {
-      var grammar = 'start = a:"a" b:"b" c:"c" !{ code }';
+      let grammar = 'start = a:"a" b:"b" c:"c" !{ code }';
 
       it("generates correct bytecode", function() {
         expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -533,7 +533,7 @@ describe("compiler pass |generateBytecode|", function() {
 
   describe("for literal", function() {
     describe("empty", function() {
-      var grammar = 'start = ""';
+      let grammar = 'start = ""';
 
       it("generates correct bytecode", function() {
         expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -547,7 +547,7 @@ describe("compiler pass |generateBytecode|", function() {
     });
 
     describe("non-empty case-sensitive", function() {
-      var grammar = 'start = "a"';
+      let grammar = 'start = "a"';
 
       it("generates correct bytecode", function() {
         expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -566,7 +566,7 @@ describe("compiler pass |generateBytecode|", function() {
     });
 
     describe("non-empty case-insensitive", function() {
-      var grammar = 'start = "A"i';
+      let grammar = 'start = "A"i';
 
       it("generates correct bytecode", function() {
         expect(pass).toChangeAST(grammar, bytecodeDetails([
@@ -632,7 +632,7 @@ describe("compiler pass |generateBytecode|", function() {
   });
 
   describe("for any", function() {
-    var grammar = 'start = .';
+    let grammar = 'start = .';
 
     it("generates bytecode", function() {
       expect(pass).toChangeAST(grammar, bytecodeDetails([
diff --git a/spec/unit/compiler/passes/helpers.js b/spec/unit/compiler/passes/helpers.js
index da0a4d2..113f2fa 100644
--- a/spec/unit/compiler/passes/helpers.js
+++ b/spec/unit/compiler/passes/helpers.js
@@ -1,6 +1,6 @@
 "use strict";
 
-var peg = require("../../../../lib/peg");
+let peg = require("../../../../lib/peg");
 
 beforeEach(function() {
   this.addMatchers({
@@ -16,7 +16,7 @@ beforeEach(function() {
           return value !== null && typeof value === "object";
         }
 
-        var i, key;
+        let i, key;
 
         if (isArray(details)) {
           if (!isArray(value)) { return false; }
@@ -44,7 +44,7 @@ beforeEach(function() {
         }
       }
 
-      var ast = peg.parser.parse(grammar);
+      let ast = peg.parser.parse(grammar);
 
       this.actual(ast, options);
 
@@ -61,7 +61,7 @@ beforeEach(function() {
     },
 
     toReportError: function(grammar, details) {
-      var ast = peg.parser.parse(grammar),
+      let ast = peg.parser.parse(grammar),
           key;
 
       try {
diff --git a/spec/unit/compiler/passes/remove-proxy-rules.spec.js b/spec/unit/compiler/passes/remove-proxy-rules.spec.js
index 5ab6691..d989a8b 100644
--- a/spec/unit/compiler/passes/remove-proxy-rules.spec.js
+++ b/spec/unit/compiler/passes/remove-proxy-rules.spec.js
@@ -1,9 +1,9 @@
 "use strict";
 
-var peg = require("../../../../lib/peg");
+let peg = require("../../../../lib/peg");
 
 describe("compiler pass |removeProxyRules|", function() {
-  var pass = peg.compiler.passes.transform.removeProxyRules;
+  let pass = peg.compiler.passes.transform.removeProxyRules;
 
   describe("when a proxy rule isn't listed in |allowedStartRules|", function() {
     it("updates references and removes it", function() {
diff --git a/spec/unit/compiler/passes/report-duplicate-labels.spec.js b/spec/unit/compiler/passes/report-duplicate-labels.spec.js
index 11d5b2f..3d8aa43 100644
--- a/spec/unit/compiler/passes/report-duplicate-labels.spec.js
+++ b/spec/unit/compiler/passes/report-duplicate-labels.spec.js
@@ -1,9 +1,9 @@
 "use strict";
 
-var peg = require("../../../../lib/peg");
+let peg = require("../../../../lib/peg");
 
 describe("compiler pass |reportDuplicateLabels|", function() {
-  var pass = peg.compiler.passes.check.reportDuplicateLabels;
+  let pass = peg.compiler.passes.check.reportDuplicateLabels;
 
   describe("in a sequence", function() {
     it("reports labels duplicate with labels of preceding elements", function() {
diff --git a/spec/unit/compiler/passes/report-duplicate-rules.spec.js b/spec/unit/compiler/passes/report-duplicate-rules.spec.js
index cdc45c5..46c31b4 100644
--- a/spec/unit/compiler/passes/report-duplicate-rules.spec.js
+++ b/spec/unit/compiler/passes/report-duplicate-rules.spec.js
@@ -1,9 +1,9 @@
 "use strict";
 
-var peg = require("../../../../lib/peg");
+let peg = require("../../../../lib/peg");
 
 describe("compiler pass |reportDuplicateRules|", function() {
-  var pass = peg.compiler.passes.check.reportDuplicateRules;
+  let pass = peg.compiler.passes.check.reportDuplicateRules;
 
   it("reports duplicate rules", function() {
     expect(pass).toReportError([
diff --git a/spec/unit/compiler/passes/report-infinite-recursion.spec.js b/spec/unit/compiler/passes/report-infinite-recursion.spec.js
index 906fd8b..6f3fb4d 100644
--- a/spec/unit/compiler/passes/report-infinite-recursion.spec.js
+++ b/spec/unit/compiler/passes/report-infinite-recursion.spec.js
@@ -1,9 +1,9 @@
 "use strict";
 
-var peg = require("../../../../lib/peg");
+let peg = require("../../../../lib/peg");
 
 describe("compiler pass |reportInfiniteRecursion|", function() {
-  var pass = peg.compiler.passes.check.reportInfiniteRecursion;
+  let pass = peg.compiler.passes.check.reportInfiniteRecursion;
 
   it("reports direct left recursion", function() {
     expect(pass).toReportError('start = start', {
diff --git a/spec/unit/compiler/passes/report-infinite-repetition.spec.js b/spec/unit/compiler/passes/report-infinite-repetition.spec.js
index 1671926..eccc349 100644
--- a/spec/unit/compiler/passes/report-infinite-repetition.spec.js
+++ b/spec/unit/compiler/passes/report-infinite-repetition.spec.js
@@ -1,9 +1,9 @@
 "use strict";
 
-var peg = require("../../../../lib/peg");
+let peg = require("../../../../lib/peg");
 
 describe("compiler pass |reportInfiniteRepetition|", function() {
-  var pass = peg.compiler.passes.check.reportInfiniteRepetition;
+  let pass = peg.compiler.passes.check.reportInfiniteRepetition;
 
   it("reports infinite loops for zero_or_more", function() {
     expect(pass).toReportError('start = ("")*', {
diff --git a/spec/unit/compiler/passes/report-undefined-rules.spec.js b/spec/unit/compiler/passes/report-undefined-rules.spec.js
index ad73007..3f4df85 100644
--- a/spec/unit/compiler/passes/report-undefined-rules.spec.js
+++ b/spec/unit/compiler/passes/report-undefined-rules.spec.js
@@ -1,9 +1,9 @@
 "use strict";
 
-var peg = require("../../../../lib/peg");
+let peg = require("../../../../lib/peg");
 
 describe("compiler pass |reportUndefinedRules|", function() {
-  var pass = peg.compiler.passes.check.reportUndefinedRules;
+  let pass = peg.compiler.passes.check.reportUndefinedRules;
 
   it("reports undefined rules", function() {
     expect(pass).toReportError('start = undefined', {
diff --git a/spec/unit/parser.spec.js b/spec/unit/parser.spec.js
index 0ef46c8..6f6a85c 100644
--- a/spec/unit/parser.spec.js
+++ b/spec/unit/parser.spec.js
@@ -1,9 +1,9 @@
 "use strict";
 
-var peg = require("../../lib/peg");
+let peg = require("../../lib/peg");
 
 describe("PEG.js grammar parser", function() {
-  var literalAbcd       = { type: "literal",      value: "abcd", ignoreCase: false },
+  let literalAbcd       = { type: "literal",      value: "abcd", ignoreCase: false },
       literalEfgh       = { type: "literal",      value: "efgh", ignoreCase: false },
       literalIjkl       = { type: "literal",      value: "ijkl", ignoreCase: false },
       literalMnop       = { type: "literal",      value: "mnop", ignoreCase: false },
@@ -96,14 +96,14 @@ describe("PEG.js grammar parser", function() {
     return oneRuleGrammar({ type: "rule_ref", name: name });
   }
 
-  var trivialGrammar = literalGrammar("abcd", false),
+  let trivialGrammar = literalGrammar("abcd", false),
       twoRuleGrammar = {
         type:        "grammar",
         initializer: null,
         rules:       [ruleA, ruleB]
       };
 
-  var stripLocation = (function() {
+  let stripLocation = (function() {
     function buildVisitor(functions) {
       return function(node) {
         return functions[node.type].apply(null, arguments);
@@ -128,7 +128,7 @@ describe("PEG.js grammar parser", function() {
       };
     }
 
-    var strip = buildVisitor({
+    let strip = buildVisitor({
       grammar: function(node) {
         delete node.location;
 
@@ -166,7 +166,7 @@ describe("PEG.js grammar parser", function() {
   beforeEach(function() {
     this.addMatchers({
       toParseAs:     function(expected) {
-        var result;
+        let result;
 
         try {
           result = peg.parser.parse(this.actual);
@@ -194,7 +194,7 @@ describe("PEG.js grammar parser", function() {
       },
 
       toFailToParse: function(details) {
-        var result, key;
+        let result, key;
 
         try {
           result = peg.parser.parse(this.actual);
diff --git a/src/parser.pegjs b/src/parser.pegjs
index 9578d94..61cc7ac 100644
--- a/src/parser.pegjs
+++ b/src/parser.pegjs
@@ -24,19 +24,19 @@
  */
 
 {
-  var OPS_TO_PREFIXED_TYPES = {
+  const OPS_TO_PREFIXED_TYPES = {
     "$": "text",
     "&": "simple_and",
     "!": "simple_not"
   };
 
-  var OPS_TO_SUFFIXED_TYPES = {
+  const OPS_TO_SUFFIXED_TYPES = {
     "?": "optional",
     "*": "zero_or_more",
     "+": "one_or_more"
   };
 
-  var OPS_TO_SEMANTIC_PREDICATE_TYPES = {
+  const OPS_TO_SEMANTIC_PREDICATE_TYPES = {
     "&": "semantic_and",
     "!": "semantic_not"
   };