diff --git a/README.md b/README.md
index aa03199..87bceb0 100644
--- a/README.md
+++ b/README.md
@@ -19,6 +19,8 @@ Docuris have many advantages:
 Give Docuris a try!
 
 ## Usage
+
+### `parse(string)`
 Parse id string:
 ```js
 var docuri = require('docuri');
@@ -32,6 +34,7 @@ docuri.parse('mytype/myid/mysubtype/myindex/myversion');
 // }
 ```
 
+### `stringify(object)`
 Build id string from object:
 ```js
 docuri.stringify({
@@ -44,6 +47,7 @@ docuri.stringify({
 // 'mytype/myid/mysubtype/myindex/myversion'
 ```
 
+### `merge(objectOrString)`
 Change id string components:
 ```js
 docuri.merge('mytype/myid/mysubtype/myindex/myversion', {
@@ -52,6 +56,25 @@ docuri.merge('mytype/myid/mysubtype/myindex/myversion', {
 // 'my_new_type/myid/mysubtype/myindex/myversion'
 ```
 
+### `parts(objectOrString)`
+Array of components. Trailing `undefined` components are stripped off:
+```js
+docuri.parts('mytype/myid/');
+// ['mytype', 'myid']
+docuri.parts({ type: 'mytype', subtype: 'mysubtype' });
+// ['mytype', undefined, 'mysubtype']
+```
+
+### `arity(objectOrString)`
+Length of parts:
+```js
+docuri.arity('mytype/myid/');
+// 2 
+docuri.arity({ type: 'mytype', subtype: 'mysubtype' });
+// 3
+```
+
+### `docuri([definition])`
 Use custom definition:
 ```js
 docuri();
@@ -65,6 +88,8 @@ docuri.parse('42/answer');
 //   meta: 'answer'
 // }
 ```
+Note: `definition` MUST be an array of strings with at least one item.
+
 
 ## Browser support
 To use docid in your client-side application, browserify it like this:
diff --git a/index.js b/index.js
index e6a8b75..67f29ae 100644
--- a/index.js
+++ b/index.js
@@ -6,25 +6,41 @@
 */
 
 // type/id/subtype/index/version
-var PARTS = ['type', 'id', 'subtype', 'index', 'version'];
+var DEFINITION = ['type', 'id', 'subtype', 'index', 'version'];
 
-function docuri(parts) {
-  if (parts) {
-    PARTS = parts;
+function docuri(definition) {
+  if (definition) {
+    DEFINITION = definition;
 
     return docuri;
   }
 
-  return PARTS;
+  return DEFINITION;
 }
 
 
+docuri.parts = function(obj) {
+  if (typeof obj === 'string') {
+    obj = docuri.parse(obj);
+  }
+
+  var parts = DEFINITION.map(function(part) {
+    return obj[part];
+  });
+
+  while (parts.length && typeof parts[parts.length - 1] === 'undefined') {
+    parts.pop();
+  }
+
+  return parts;
+};
+
 docuri.parse = function(str) {
   str = str || '';
 
   return str.split('/').reduce(function(obj, value, i) {
     if (value) {
-      obj[PARTS[i]] = value;
+      obj[DEFINITION[i]] = value;
     }
 
     return obj;
@@ -34,17 +50,17 @@ docuri.parse = function(str) {
 docuri.stringify = function(obj) {
   obj = obj || {};
 
-  return PARTS.map(function(part) {
-    return typeof obj[part] === 'undefined' ? '' : obj[part];
-  }).join('/').replace(/\/+$/, '');
+  return docuri.parts(obj).join('/');
 };
 
-docuri.merge = function(str, objToMerge) {
+docuri.merge = function(obj, objToMerge) {
   objToMerge = objToMerge || {};
 
-  var obj = docuri.parse(str);
+  if (typeof obj === 'string') {
+    obj = docuri.parse(obj);
+  }
 
-  PARTS.forEach(function(part) {
+  DEFINITION.forEach(function(part) {
     if (objToMerge[part]) {
       obj[part] = objToMerge[part];
     }
@@ -53,6 +69,14 @@ docuri.merge = function(str, objToMerge) {
   return docuri.stringify(obj);
 };
 
+docuri.arity = function(obj) {
+  if (typeof obj === 'string') {
+    obj = docuri.parse(obj);
+  }
+
+  return docuri.parts(obj).length;
+};
+
 
 module.exports = docuri;
 
diff --git a/package.json b/package.json
index 1a43555..9a9be9b 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "docuri",
-  "version": "2.1.0",
+  "version": "2.2.0",
   "description": "Rich document ids for CouchDB",
   "main": "index.js",
   "scripts": {
diff --git a/test/arity_test.js b/test/arity_test.js
new file mode 100644
index 0000000..d8c4a06
--- /dev/null
+++ b/test/arity_test.js
@@ -0,0 +1,36 @@
+var test = require('tap').test;
+var arity = require('..').arity;
+
+test('arity of type string', function(t) {
+  t.equal(arity('type'), 1, 'should return 1');
+  t.end();
+});
+test('arity of type/id string', function(t) {
+  t.equal(arity('type/id'), 2, 'should return 2');
+  t.end();
+});
+test('arity of type/id/subtype string', function(t) {
+  t.equal(arity('type/id/subtype'), 3, 'should return 3');
+  t.end();
+});
+test('arity of type//subtype string', function(t) {
+  t.equal(arity('type//subtype'), 3, 'should return 3');
+  t.end();
+});
+
+test('arity of type object', function(t) {
+  t.equal(arity({ type: 'type' }), 1, 'should return 1');
+  t.end();
+});
+test('arity of type/id object', function(t) {
+  t.equal(arity({ type: 'type', id: 'id'}), 2, 'should return 2');
+  t.end();
+});
+test('arity of type/id/subtype object', function(t) {
+  t.equal(arity({ type: 'type', id: 'id', subtype: 'subtype' }), 3, 'should return 3');
+  t.end();
+});
+test('arity of type//subtype object', function(t) {
+  t.equal(arity({ type: 'type', subtype: 'subtype' }), 3, 'should return 3');
+  t.end();
+});
diff --git a/test/config_test.js b/test/config_test.js
new file mode 100644
index 0000000..8badff8
--- /dev/null
+++ b/test/config_test.js
@@ -0,0 +1,28 @@
+var test = require('tap').test;
+var docuri = require('..');
+
+test('default configuration', function(t) {
+  t.deepEqual(docuri(), ['type', 'id', 'subtype', 'index', 'version'], 'should return default parts');
+  t.end();
+});
+
+test('set configuration', function(t) {
+  var parts = ['my', 'parts'];
+  t.type(docuri(parts).merge, 'function', 'should return docuri api: merge');
+  t.type(docuri(parts).parse, 'function', 'should return docuri api: parse');
+  t.type(docuri(parts).stringify, 'function', 'should return docuri api: stringify');
+  t.end();
+});
+
+test('change configuration', function(t) {
+  var parts = ['my', 'parts'];
+  docuri(parts);
+  t.deepEqual(docuri(), parts, 'should return custom parts');
+  t.end();
+});
+
+test('use changed configuration', function(t) {
+  var parts = ['my', 'parts'];
+  t.deepEqual(docuri(parts).parse('one/two'), { my: 'one', parts: 'two'}, 'should use custom parts');
+  t.end();
+});
diff --git a/test/merge_test.js b/test/merge_test.js
index 989522e..4386968 100644
--- a/test/merge_test.js
+++ b/test/merge_test.js
@@ -38,3 +38,14 @@ test('missing second argument', function(t) {
 });
 
 
+test('changing type component using object', function(t) {
+  t.equal(merge({
+    type: 'type',
+    id: 'id',
+    subtype: 'subtype',
+    index: 'index',
+    version: 'version'
+  }, {type:'new_type'}), 'new_type/id/subtype/index/version', 'should return docuri string with type changed');
+  t.end();
+});
+
diff --git a/test/parts_test.js b/test/parts_test.js
index 8badff8..b88bd7b 100644
--- a/test/parts_test.js
+++ b/test/parts_test.js
@@ -1,28 +1,36 @@
 var test = require('tap').test;
-var docuri = require('..');
+var parts = require('..').parts;
 
-test('default configuration', function(t) {
-  t.deepEqual(docuri(), ['type', 'id', 'subtype', 'index', 'version'], 'should return default parts');
+test('parts of type string', function(t) {
+  t.deepEqual(parts('type'), ['type'], 'should return array including type');
   t.end();
 });
-
-test('set configuration', function(t) {
-  var parts = ['my', 'parts'];
-  t.type(docuri(parts).merge, 'function', 'should return docuri api: merge');
-  t.type(docuri(parts).parse, 'function', 'should return docuri api: parse');
-  t.type(docuri(parts).stringify, 'function', 'should return docuri api: stringify');
+test('parts of type/id string', function(t) {
+  t.deepEqual(parts('type/id'), ['type', 'id'], 'should return array including type and id');
   t.end();
 });
-
-test('change configuration', function(t) {
-  var parts = ['my', 'parts'];
-  docuri(parts);
-  t.deepEqual(docuri(), parts, 'should return custom parts');
+test('parts of type/id/subtype string', function(t) {
+  t.deepEqual(parts('type/id/subtype'), ['type', 'id', 'subtype'], 'should return array including type, id and subtype');
+  t.end();
+});
+test('parts of type//subtype string', function(t) {
+  t.deepEqual(parts('type//subtype'), ['type', undefined, 'subtype'], 'should return array including type, undefined and subtype');
   t.end();
 });
 
-test('use changed configuration', function(t) {
-  var parts = ['my', 'parts'];
-  t.deepEqual(docuri(parts).parse('one/two'), { my: 'one', parts: 'two'}, 'should use custom parts');
+test('parts of type object', function(t) {
+  t.deepEqual(parts({ type: 'type' }), ['type'], 'should return array including type');
+  t.end();
+});
+test('parts of type/id object', function(t) {
+  t.deepEqual(parts({ type: 'type', id: 'id'}), ['type', 'id'], 'should return array including type and id');
+  t.end();
+});
+test('parts of type/id/subtype object', function(t) {
+  t.deepEqual(parts({ type: 'type', id: 'id', subtype: 'subtype' }), ['type', 'id', 'subtype'], 'should return array including type, id and subtype');
+  t.end();
+});
+test('parts of type//subtype object', function(t) {
+  t.deepEqual(parts({ type: 'type', subtype: 'subtype' }), ['type', undefined, 'subtype'], 'should return array including type, undefined and subtype');
   t.end();
 });