Symbol.iterator
The well-known Symbol.iterator
symbol specifies the default iterator for an object. Used by for...of
.
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:
Array.prototype[@@iterator]()
TypedArray.prototype[@@iterator]()
String.prototype[@@iterator]()
Map.prototype[@@iterator]()
Set.prototype[@@iterator]()
See also Iteration protocols for more information.
Property attributes of Symbol.iterator |
|
---|---|
Writable | no |
Enumerable | no |
Configurable | no |
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]
Or iterables can be defined directly inside a class or object using a computed property:
class Foo {
*[Symbol.iterator] () {
yield 1;
yield 2;
yield 3;
}
}
const someObj = {
*[Symbol.iterator] () {
yield 'a';
yield 'b';
}
}
console.log(...new Foo); // 1, 2, 3
console.log(...someObj); // 'a', 'b'
Non-well-formed iterables
If an iterable's @@iterator
method does not return an iterator object, then it is 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 |
---|
ECMAScript Language Specification # sec-symbol.iterator |
Browser compatibility
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