A couple of additions to ECMAScript 2015 (ES6) aren't new built-ins or syntax, but protocols. These protocols can be implemented by any object respecting some conventions.

There are two protocols: The iterable protocol and the iterator protocol.

The iterable protocol

The iterable protocol allows JavaScript objects to define or customize their iteration behavior, such as what values are looped over in a for..of construct. Some built-in types are built-in iterables with a default iteration behavior, such as Array or Map, while other types (such as Object) are not.

In order to be iterable, an object must implement the @@iterator method, meaning that the object (or one of the objects up its prototype chain) must have a property with a @@iterator key which is available via constant Symbol.iterator:

Property Value
[Symbol.iterator] A zero arguments function that returns an object, conforming to the iterator protocol.

Whenever an object needs to be iterated (such as at the beginning of a for..of loop), its @@iterator method is called with no arguments, and the returned iterator is used to obtain the values to be iterated.

The iterator protocol

The iterator protocol defines a standard way to produce a sequence of values (either finite or infinite).

An object is an iterator when it implements a next() method with the following semantics:

Property Value
next

A zero arguments function that returns an object with two properties:

  • done (boolean)
    • Has the value true if the iterator is past the end of the iterated sequence. In this case value optionally specifies the return value of the iterator. The return values are explained here.
    • Has the value false if the iterator was able to produce the next value in the sequence. This is equivalent of not specifying the done property altogether.
  • value - any JavaScript value returned by the iterator. Can be omitted when done is true.

The next method always has to return an object with appropriate properties including done and value. If a non-object value gets returned (such as false or undefined), a TypeError ("iterator.next() returned a non-object value") will be thrown.

Some iterators are in turn iterables:

var someArray = [1, 5, 7];
var someArrayEntries = someArray.entries();

someArrayEntries.toString();           // "[object Array Iterator]"
someArrayEntries === someArrayEntries[Symbol.iterator]();    // true

Examples using the iteration protocols

A String is an example of a built-in iterable object:

var someString = "hi";
typeof someString[Symbol.iterator];          // "function"

String's default iterator returns the string's characters one by one:

var iterator = someString[Symbol.iterator]();
iterator + "";                               // "[object String Iterator]"
 
iterator.next();                             // { value: "h", done: false }
iterator.next();                             // { value: "i", done: false }
iterator.next();                             // { value: undefined, done: true }

Some built-in constructs, such as the spread operator, use the same iteration protocol under the hood:

[...someString]                              // ["h", "i"]

We can redefine the iteration behavior by supplying our own @@iterator:

var someString = new String("hi");          // need to construct a String object explicitly to avoid auto-boxing

someString[Symbol.iterator] = function() {
  return { // this is the iterator object, returning a single element, the string "bye"
    next: function() {
      if (this._first) {
        this._first = false;
        return { value: "bye", done: false };
      } else {
        return { done: true };
      }
    },
    _first: true
  };
};

Notice how redefining @@iterator affects the behavior of built-in constructs, that use the iteration protocol:

[...someString];                              // ["bye"]
someString + "";                              // "hi"

Iterable examples

Builtin iterables

String, Array, TypedArray, Map and Set are all built-in iterables, because the prototype objects of them all have an @@iterator method.

User-defined iterables

We can make our own iterables like this:

var myIterable = {};
myIterable[Symbol.iterator] = function* () {
    yield 1;
    yield 2;
    yield 3;
};
[...myIterable]; // [1, 2, 3]

Builtin APIs accepting iterables

There are many APIs accepting iterables, for example: Map([iterable]), WeakMap([iterable]), Set([iterable]) and WeakSet([iterable]):

var myObj = {};
new Map([[1,"a"],[2,"b"],[3,"c"]]).get(2);               // "b"
new WeakMap([[{},"a"],[myObj,"b"],[{},"c"]]).get(myObj); // "b"
new Set([1, 2, 3]).has(3);                               // true
new Set("123").has("2");                                 // true
new WeakSet(function*() {
    yield {};
    yield myObj;
    yield {};
}()).has(myObj);                                         // true

But also Promise.all(iterable), Promise.race(iterable), and Array.from().

Syntaxes expecting iterables

Some statements and expressions are expecting iterables, for example the for-of loops, spread operator, yield*, and destructuring assignment.

for(let value of ["a", "b", "c"]){
    console.log(value);
}
// "a"
// "b"
// "c"

[..."abc"]; // ["a", "b", "c"]

function* gen(){
  yield* ["a", "b", "c"];
}

gen().next(); // { value:"a", done:false }

[a, b, c] = new Set(["a", "b", "c"]);
a // "a"

Non-well-formed iterables

If an iterable's @@iterator method doesn't return an iterator object, then it's a non-well-formed iterable, using it as such is likely to result in runtime exceptions or buggy behavior:

var nonWellFormedIterable = {}
nonWellFormedIterable[Symbol.iterator] = () => 1
[...nonWellFormedIterable] // TypeError: [] is not a function

Iterator examples

Simple iterator

function makeIterator(array){
    var nextIndex = 0;
    
    return {
       next: function(){
           return nextIndex < array.length ?
               {value: array[nextIndex++], done: false} :
               {done: true};
       }
    };
}

var it = makeIterator(['yo', 'ya']);

console.log(it.next().value); // 'yo'
console.log(it.next().value); // 'ya'
console.log(it.next().done);  // true

Infinite iterator

function idMaker(){
    var index = 0;
    
    return {
       next: function(){
           return {value: index++, done: false};
       }
    };
}

var it = idMaker();

console.log(it.next().value); // '0'
console.log(it.next().value); // '1'
console.log(it.next().value); // '2'
// ...

With a generator

function* makeSimpleGenerator(array){
    var nextIndex = 0;
    
    while(nextIndex < array.length){
        yield array[nextIndex++];
    }
}

var gen = makeSimpleGenerator(['yo', 'ya']);

console.log(gen.next().value); // 'yo'
console.log(gen.next().value); // 'ya'
console.log(gen.next().done);  // true



function* idMaker(){
    var index = 0;
    while(true)
        yield index++;
}

var gen = idMaker();

console.log(gen.next().value); // '0'
console.log(gen.next().value); // '1'
console.log(gen.next().value); // '2'
// ...

Is a generator object an iterator or an iterable?

A generator object is both, iterator and iterable:

var aGeneratorObject = function*(){
    yield 1;
    yield 2;
    yield 3;
}();
typeof aGeneratorObject.next;
// "function", because it has a next method, so it's an iterator
typeof aGeneratorObject[Symbol.iterator];
// "function", because it has an @@iterator method, so it's an iterable
aGeneratorObject[Symbol.iterator]() === aGeneratorObject;
// true, because its @@iterator method return its self (an iterator), so it's an well-formed iterable
[...aGeneratorObject];
// [1, 2, 3]

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 39.0 27.0 (27.0) No support 26 No support
IteratorResult object instead of throwing (Yes) 29.0 (29.0) No support (Yes) No support
Feature Android Android Webview Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile Chrome for Android
Basic support No support (Yes) 27.0 (27.0) No support No support No support 39.0
IteratorResult object instead of throwing No support ? 29.0 (29.0) No support No support No support (Yes)

Firefox-specific notes

IteratorResult object returned instead of throwing

Starting with Gecko 29 (Firefox 29 / Thunderbird 29 / SeaMonkey 2.26), the completed generator function no longer throws a TypeError "generator has already finished". Instead, it returns an IteratorResult object like { value: undefined, done: true } (bug 958951).

Iterator property and @@iterator symbol

From Gecko 17 (Firefox 17 / Thunderbird 17 / SeaMonkey 2.14) to Gecko 26 (Firefox 26 / Thunderbird 26 / SeaMonkey 2.23 / Firefox OS 1.2) the iterator property was used (bug 907077), and from Gecko 27 to Gecko 35 the "@@iterator" placeholder was used. In Gecko 36 (Firefox 36 / Thunderbird 36 / SeaMonkey 2.33), the @@iterator symbol got implemented (bug 918828).

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Iteration' in that specification.
Standard Initial definition.
ECMAScript 2017 Draft (ECMA-262)
The definition of 'Iteration' in that specification.
Draft  

See also

Document Tags and Contributors

 Last updated by: fscholz,