The forEach()
method executes a provided function once per each key/value pair in the Map
object, in insertion order.
Syntax
myMap.forEach(callback[, thisArg])
Parameters
callback
- Function to execute for each element.
thisArg
- Value to use as
this
when executingcallback
.
Return value
Description
The forEach
method executes the provided callback
once for each key of the map which actually exist. It is not invoked for keys which have been deleted. However, it is executed for values which are present but have the value undefined
.
callback
is invoked with three arguments:
- the element value
- the element key
- the
Map
object being traversed
If a thisArg
parameter is provided to forEach
, it will be passed to callback
when invoked, for use as its this
value. Otherwise, the value undefined
will be passed for use as its this
value. The this
value ultimately observable by callback
is determined according to the usual rules for determining the this
seen by a function.
Each value is visited once, except in the case when it was deleted and re-added before forEach
has finished. callback
is not invoked for values deleted before being visited. New values added before forEach
has finished will be visited.
forEach
executes the callback
function once for each element in the Map
object; it does not return a value.
Examples
Printing the contents of a Map
object
The following code logs a line for each element in an Map
object:
function logMapElements(value, key, map) { console.log(`m[${key}] = ${value}`); } new Map([['foo', 3], ['bar', {}], ['baz', undefined]]).forEach(logMapElements); // logs: // "m[foo] = 3" // "m[bar] = [object Object]" // "m[baz] = undefined"
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Map.prototype.forEach' in that specification. |
Standard | Initial definition. |
ECMAScript Latest Draft (ECMA-262) The definition of 'Map.prototype.forEach' in that specification. |
Living Standard |
Browser compatibility
Feature | Chrome | Edge | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|---|
Basic support | 38 | (Yes) | 25.0 (25.0) | 11 | 25 | 7.1 |
Feature | Android | Chrome for Android | Edge | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|---|
Basic support | No support | 38 | (Yes) | 25.0 (25.0) | No support | No support | 8 |