Symbol.iterator

The Symbol.iterator well-known symbol specifies the default iterator for an object. Used by for...of.

Property attributes of Symbol.iterator
Writable no
Enumerable no
Configurable no

Description

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.

Some built-in types have a default iteration behavior, while other types (such as Object) do not. The built-in types with a @@iterator method are:

See also Iteration protocols for more information.

Examples

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]

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

Specifications

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

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 43 36 (36) No support 30 10
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support No support No support 36.0 (36) No support No support No support

See also

Document Tags and Contributors

 Contributors to this page: mhthompson86, fscholz, DevelX
 Last updated by: mhthompson86,