在此页

• 句法
• 描述
• 范例
• 规范
• 浏览器兼容性
• 另请参阅
• 相关话题

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.

句法

myMap.forEach(callback([value][,key][,map])[, thisArg])
					

参数

callback

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

value 可选

Value of each iteration.

key 可选

Key of each iteration.

map 可选

The map being iterated ( myMap in the above Syntax box).

thisArg 可选

Value to use as this when executing callback .

返回值

undefined .

描述

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

若 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.

范例

Printing the contents of a Map object

The following code logs a line for each element in an Map 对象：

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"
					

规范

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

浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

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

图例

完整支持

完整支持

另请参阅

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

元数据

• 最后修改： Aug 2, 2020

相关话题

1. 标准内置对象
2. Map
3. 特性
   1. Map.prototype.size
   2. Map.prototype[@@toStringTag]
   3. get Map[@@species]
4. 方法
   1. Map.prototype.clear()
   2. Map.prototype.delete()
   3. Map.prototype.entries()
   4. Map.prototype.forEach()
   5. Map.prototype.get()
   6. Map.prototype.has()
   7. Map.prototype.keys()
   8. Map.prototype.set()
   9. Map.prototype.values()
   10. Map.prototype[@@iterator]()
5. 继承：
6. Function
7. 特性
   1. Function.arguments
   2. Function.caller
   3. Function.displayName
   4. Function.length
   5. Function.name
8. 方法
   1. Function.prototype.apply()
   2. Function.prototype.bind()
   3. Function.prototype.call()
   4. Function.prototype.toSource()
   5. Function.prototype.toString()
9. Object
10. 特性
    1. Object.prototype.__proto__
    2. Object.prototype.constructor
11. 方法
    1. Object.prototype.__defineGetter__()
    2. Object.prototype.__defineSetter__()
    3. Object.prototype.__lookupGetter__()
    4. Object.prototype.__lookupSetter__()
    5. Object.prototype.hasOwnProperty()
    6. Object.prototype.isPrototypeOf()
    7. Object.prototype.propertyIsEnumerable()
    8. Object.prototype.toLocaleString()
    9. Object.prototype.toSource()
    10. Object.prototype.toString()
    11. Object.prototype.valueOf()
    12. Object.setPrototypeOf()