javascript forEach() 方法讲解
<strong>forEach()</strong>方法让数组的每一项都执行一次给定的函数。
<strong>forEach()基本语法</strong>
<code class="javascript">array.forEach(callback[, thisArg])</code>
<strong>forEach()参数介绍</strong>
| 参数名 | 介绍 |
| callback |
在数组每一项上执行的函数，接收三个参数：
|
| thisArg | 可选参数。用来当作callback 函数内this的值的对象。 |
<strong>forEach()功能说明：</strong>
forEach 方法按升序为数组中含有效值的每一项执行一次callback 函数，那些已删除（使用delete方法等情况）或者从未赋值的项将被跳过（但不包括哪些值为 undefined 的项）。
callback 函数会被依次传入三个参数：
- 数组当前项的值
- 数组当前项的索引
- 数组对象本身
如果给forEach传递了thisArg 参数，它将作为 callback 函数的执行上下文，类似执行如下函数callback.call(thisArg, element, index, array)。如果 thisArg 值为 undefined 或 null，函数的this 值取决于当前执行环境是否为严格模式（严格模式下为 undefined，非严格模式下为全局对象）。
forEach 遍历的范围在第一次调用 callback 前就会确定。调用forEach 后添加到数组中的项不会被 callback 访问到。如果已经存在的值被改变，则传递给 callback 的值是 forEach 遍历到他们那一刻的值。已删除的项不会被遍历到。
注意： 没有办法中止或者跳出 forEach 循环，除了抛出一个异常。如果你需要这样，使用forEach()方法是错误的，你可以用一个简单的循环作为替代。如果您正在测试一个数组里的元素是否符合某条件，且需要返回一个布尔值，那么可使用 Array.every 或 Array.some。
forEach 为数组中的元素执行一次 callback 函数，不像 every 和 some，它总是返回 undefined。
<strong>forEach()实例一：</strong>打印出数组的内容
下面的代码会为每一个数组元素输出一行记录：
<code class="javascript">function logArrayElements(element, index, array) {
console.log("a[" + index + "] = " + element);
}
[2, 5, 9].forEach(logArrayElements);
// logs:
// a[0] = 2
// a[1] = 5
// a[2] = 9</code>
<strong>forEach()实例二：</strong>对象复制函数
下面的代码会创建一个给定对象的副本。 创建对象的副本有不同的方法，以下是只是一种方法，并解释了Array.prototype.forEach() 是如何使用ECMAScript 5 Object.* 元属性（meta property ）函数工作的。
<code class="javascript">function copy(o) {
var copy = Object.create(Object.getPrototypeOf(o));
var propNames = Object.getOwnPropertyNames(o);
propNames.forEach(function(name) {
var desc = Object.getOwnPropertyDescriptor(o, name);
Object.defineProperty(copy, name, desc);
});
return copy;
}
var o1 = { a: 1, b: 2 };
var o2 = copy(o1); // o2 looks like o1 now</code>
forEach()兼容旧环境
forEach 是在第五版本里被添加到 ECMA-262 标准的；这样它可能在标准的其他实现中不存在，你可以在你调用 forEach 之前 插入下面的代码，在本地不支持的情况下使用 forEach()。该算法是 ECMA-262 第5版中指定的算法。算法假定Object和TypeError拥有它们的初始值。callback.call 等价于Function.prototype.call()。
<code class="javascript">// Production steps of ECMA-262, Edition 5, 15.4.4.18
// Reference: http://es5.github.com/#x15.4.4.18
if ( !Array.prototype.forEach ) {
Array.prototype.forEach = function forEach( callback, thisArg ) {
var T, k;
if ( this == null ) {
throw new TypeError( "this is null or not defined" );
}
// 1. Let O be the result of calling ToObject passing the |this| value as the argument.
var O = Object(this);
// 2. Let lenValue be the result of calling the Get internal method of O with the argument "length".
// 3. Let len be ToUint32(lenValue).
var len = O.length >>> 0;
// 4. If IsCallable(callback) is false, throw a TypeError exception.
// See: http://es5.github.com/#x9.11
if ( typeof callback !== "function" ) {
throw new TypeError( callback + " is not a function" );
}
// 5. If thisArg was supplied, let T be thisArg; else let T be undefined.
if ( arguments.length > 1 ) {
T = thisArg;
}
// 6. Let k be 0
k = 0;
// 7. Repeat, while k </code>
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