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map.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([value][, key][, map])[, thisArg])

Parameters

callback

Function to execute for each entry of myMap. It takes the following arguments:

value Optional
Value of each iteration.
key Optional
Key of each iteration.
map Optional
The map being iterated (myMap in the above Syntax box).
thisArg Optional
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 entry's value
  • the entry's key
  • the Map 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.

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(`map.get('${key}') = ${value}`)
}
new Map([['foo', 3], ['bar', {}], ['baz', undefined]]).forEach(logMapElements)
// logs:
// "map.get('foo') = 3"
// "map.get('bar') = [object Object]"
// "map.get('baz') = undefined"

Specifications

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
forEach 38 12 25 11 25 8
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
forEach 38 38 25 25 8 3.0
Server
Node.js
forEach 0.12

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/forEach