Map.prototype.forEach() - JavaScript 编辑

The forEach() method executes a provided function once per each key/value pair in the Map object, in insertion order.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

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 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.

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

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

Browser compatibility

BCD tables only load in the browser

See also

• Array.prototype.forEach()
• Set.prototype.forEach()