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As a couple of additions to ECMAScript 2015, Iteration protocols aren't new built-ins or syntax, but protocols. These protocols can be implemented by any object by simply following 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-argument 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.

Note that when this zero-argument function is called, it is invoked as a method on the iterable object. Therefore inside of the function, the this keyword can be used to access the properties of the iterable object, to decide what to provide during the iteration.

This function can be an ordinary function, or it can be a generator function, so that when invoked, an iterator object is returned. Inside of this generator function, each entry can be provided by using yield.

The iterator protocol

The iterator protocol defines a standard way to produce a sequence of values (either finite or infinite), and potentially a return value when all values have been generated.

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

Property Value
next()

A zero-argument function that returns an object with at least the following two properties:

done (boolean)

Has the value false if the iterator was able to produce the next value in the sequence. (This is equivalent to not specifying the done property altogether.)

Has the value true if the iterator has completed its sequence. In this case, value optionally specifies the return value of the iterator.

value
Any JavaScript value returned by the iterator. Can be omitted when done is true.

The next() method must always 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.

Note: It is not possible to know reflectively whether a particular object implements the iterator protocol. However, it is easy to create an object that satisfies both the iterator and iterable protocols (as shown in the example below).

Doing so allows an iterator to be consumed by the various syntaxes expecting iterables. Thus, it is seldom useful to implement the Iterator Protocol without also implementing Iterable.

// Satisfies both the Iterator Protocol and Iterable
let myIterator = {
    next: function() {
        // ...
    },
    [Symbol.iterator]: function() { return this; }
};

Examples using the iteration protocols

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

let someString = 'hi';
console.log(typeof someString[Symbol.iterator]); // "function"

String's default iterator returns the string's code points one by one:

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

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

console.log([...someString]); // ["h", "i"]

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

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

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

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

console.log([...someString]); // ["bye"]
console.log(someString + ''); // "hi"

Iterable examples

Built-in iterables

String, Array, TypedArray, Map, and Set are all built-in iterables, because each of their prototype objects implements an @@iterator method.

User-defined iterables

You can make your own iterables like this:

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

Built-in APIs accepting iterables

There are many APIs that accept iterables. Some examples include:

new Map([[1, 'a'], [2, 'b'], [3, 'c']]).get(2); // "b"

let myObj = {};

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

See Also

Syntaxes expecting iterables

Some statements and expressions expect iterables, for example the for...of loops, the spread operator), yield*, and destructuring assignment:

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

console.log([...'abc']);   // ["a", "b", "c"]

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

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

[a, b, c] = new Set(['a', 'b', 'c']);
console.log(a);            // "a"

Non-well-formed iterables

If an iterable's @@iterator method doesn't return an iterator object, then it's considered a non-well-formed iterable.

Using one is likely to result in runtime errors or buggy behavior:

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

Iterator examples

Simple iterator

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

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

let 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) {
  let nextIndex = 0;
  while (nextIndex < array.length) {
    yield array[nextIndex++];
  }
}

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

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

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

let gen = idMaker()

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

With ES2015 class

class SimpleClass {
  constructor(data) {
    this.data = data;
  }

  [Symbol.iterator]() {
    // Use a new index for each iterator. This makes multiple
    // iterations over the iterable safe for non-trivial cases,
    // such as use of break or nested looping over the same iterable.
    let index = 0;

    return {
      next: () => {
        if (index < this.data.length) {
          return {value: this.data[index++], done: false}
        } else {
          return {done: true}
        }
      }
    }
  }
}

const simple = new SimpleClass([1,2,3,4,5]);

for (const val of simple) {
  console.log(val); // '1' '2' '3' '4' '5' 
}

Is a generator object an iterator or an iterable?

A generator object is both iterator and iterable:

let aGeneratorObject = function* () {
  yield 1;
  yield 2;
  yield 3;
}();

console.log(typeof aGeneratorObject.next);
// "function", because it has a next method, so it's an iterator

console.log(typeof aGeneratorObject[Symbol.iterator]);
// "function", because it has an @@iterator method, so it's an iterable

console.log(aGeneratorObject[Symbol.iterator]() === aGeneratorObject);
// true, because its @@iterator method returns itself (an iterator), so it's an well-formed iterable

console.log([...aGeneratorObject]);
// [1, 2, 3]

console.log(Symbol.iterator in aGeneratorObject)
// true, because @@iterator method is a property of aGeneratorObject

Specifications

Specification
ECMAScript (ECMA-262)
The definition of 'Iteration' in that specification.

See also