Map.prototype.forEach()

¥Syntax

forEach(callbackFn)
forEach(callbackFn, thisArg)

¥Parameters

callbackFn

为地图中的每个条目执行的函数。使用以下参数调用该函数：

value

每次迭代的值。

key

每次迭代的关键。

map

地图正在迭代。

thisArg Optional

执行 callbackFn 时用作 this 的值。

¥Return value

无 (undefined)。

¥None (undefined).

¥Description

forEach 方法对实际存在的映射的每个键执行一次提供的 callback。对于已删除的键，不会调用它。然而，它是针对存在但具有值 undefined 的值执行的。

¥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 使用三个参数调用：

¥callback is invoked with three arguments:

• 该条目的 value
• 该条目的 key
• 正在遍历的 Map 对象

如果将 thisArg 参数提供给 forEach，则在调用时该参数将传递给 callback，用作其 this 值。否则，值 undefined 将被传递用作其 this 值。callback 最终可观测到的 this 值根据 确定函数所见的 this 的常用规则 确定。

¥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.

每个值都会被访问一次，除非在 forEach 完成之前删除并重新添加该值。对于访问之前删除的值，不会调用 callback。将访问 forEach 完成之前添加的新值。

¥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

以下代码为 Map 对象中的每个元素记录一行：

¥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"

¥See also

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