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Map.prototype.forEach()

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 executing callback.

Return value

undefined.

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 2017 Draft (ECMA-262)
The definition of 'Map.prototype.forEach' in that specification.
Draft  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 38 25.0 (25.0) 11 25 7.1
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support No support 38 25.0 (25.0) No support No support 8

See also

Document Tags and Contributors

 Contributors to this page: eduardoboucas, janmr, fscholz, fatfisz, indolering, realityking
 Last updated by: eduardoboucas,