Array forEach 方法-优快云博客

https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/forEach

Summary

Executes a provided function once per array element.

Method of Array
Implemented in:	JavaScript 1.6 (Gecko 1.8b2 and later)
ECMAScript Edition:	ECMA-262 Edition 5

Syntax

array
.forEach(callback
[, thisObject
]);

Parameters

callback

Function to execute for each element.

thisObject

Object to use as this when executing callback .

Description

forEach executes the provided function (callback ) once for each element present in the array. callback is invoked only for indexes of the array which have assigned values; it is not invoked for indexes which have been deleted or which have never been assigned values.

callback is invoked with three arguments: the value of the element, the index of the element, and the Array object being traversed.

If a thisObject parameter is provided to forEach , it will be used as the this for each invocation of the callback . If it is not provided, or is null , the global object associated with callback is used instead.

forEach does not mutate the array on which it is called.

The range of elements processed by forEach is set before the first invocation of callback . Elements which are appended to the array after the call to forEach begins will not be visited bycallback . If existing elements of the array are changed, or deleted, their value as passed to callback will be the value at the time forEach visits them; elements that are deleted are not visited.

Compatibility

forEach is a recent addition to the ECMA-262 standard; as such it may not be present in other implementations of the standard. You can work around this by inserting the following code at the beginning of your scripts, allowing use of forEach in implementations which do not natively support it. This algorithm is exactly the one specified in ECMA-262, 5th edition, assuming Object and TypeError have their original values and that fun.call evaluates to the original value of Function.prototype.call .

if (!Array.prototype.forEach)

{

  Array.prototype.forEach = function(fun /*, thisp */)

  {

    "use strict";



    if (this === void 0 || this === null)

      throw new TypeError();



    var t = Object(this);

    var len = t.length >>> 0;

    if (typeof fun !== "function")

      throw new TypeError();



    var thisp = arguments[1];

    for (var i = 0; i < len; i++)

    {

      if (i in t)

        fun.call(thisp, t[i], i, t);

    }

  };

}

Examples

Example: Printing the contents of an array

The following code prints a line for each element in an array:

function printElt(element, index, array) {

    print("[" + index + "] is " + element); // assumes print is already defined

}

[2, 5, 9].forEach(printElt);

// Prints:

// [0] is 2

// [1] is 5

// [2] is 9

Example: Printing the contents of an array with an object method

The following code creates a simple writer object and then uses the writeln method to write one line per element in the array:

var writer = {

    sb:       [],

    write:    function (s) {

        this.sb.push(s);

    },

    writeln:  function (s) {

        this.write(s + "/n");

    },

    toString: function () {

        return this.sb.join("");

    }

};



[2, 5, 9].forEach(writer.writeln, writer);

print(writer.toString()); // assumes print is already defined

// Prints:

// 2

// 5

// 9