diff --git a/lib/compiler/passes/generate-javascript.js b/lib/compiler/passes/generate-javascript.js
index 5d5c384..206ed15 100644
--- a/lib/compiler/passes/generate-javascript.js
+++ b/lib/compiler/passes/generate-javascript.js
@@ -836,10 +836,10 @@ module.exports = function(ast, options) {
     '    }',
     '',
     '    function peg$computePosDetails(pos) {',
-    '      function advance(details, pos) {',
+    '      function advance(details, startPos, endPos) {',
     '        var p, ch;',
     '',
-    '        for (p = 0; p < pos; p++) {',
+    '        for (p = startPos; p < endPos; p++) {',
     '          ch = input.charAt(p);',
     '          if (ch === "\\n") {',
     '            if (!details.seenCR) { details.line++; }',
@@ -861,8 +861,8 @@ module.exports = function(ast, options) {
     '          peg$cachedPos = 0;',
     '          peg$cachedPosDetails = { line: 1, column: 1, seenCR: false };',
     '        }',
+    '        advance(peg$cachedPosDetails, peg$cachedPos, pos);',
     '        peg$cachedPos = pos;',
-    '        advance(peg$cachedPosDetails, peg$cachedPos);',
     '      }',
     '',
     '      return peg$cachedPosDetails;',