Array.prototype.forEach替代实现参数 [英] Array.prototype.forEach alternative implementation parameters

问题描述

在处理我最新的Web应用程序并需要使用 Array.forEach 函数时，我经常发现以下代码用于添加对没有的旧浏览器的支持内置的功能。

When working on my latest web application and needing to use the Array.forEach function, I constantly found the following code used to add support to older browsers that do not have the function built in.

/**
 * Copyright (c) Mozilla Foundation http://www.mozilla.org/
 * This code is available under the terms of the MIT License
 */
if (!Array.prototype.forEach) {
    Array.prototype.forEach = function(fun /*, thisp*/) {
        var len = this.length >>> 0;
        if (typeof fun != "function") {
            throw new TypeError();
        }

        var thisp = arguments[1];
        for (var i = 0; i < len; i++) {
            if (i in this) {
                fun.call(thisp, this[i], i, this);
            }
        }
    };
}

我完全理解代码的作用及其工作原理，但我总是看到它使用正式的 thisp 参数进行复制，并使用 arguments [1] 将其设置为局部变量。

I fully understand what the code does and how it works, but I always see it copied with the formal thisp parameter commented out and having it be set as a local variable using arguments[1] instead.

我想知道是否有人知道为什么要进行此更改，因为据我所知，代码可以正常使用 thisp 作为形式参数而不是变量？

I was wondering if anyone knew why this change was made, because from what I can tell, the code would have worked fine with thisp as a formal parameter rather than a variable?

推荐答案

Array.prototype。 forEach.length 定义为 1 ，因此如果实现函数的 .length <，则实现函数将更像本机函数/ code>属性也设置为 1 。

Array.prototype.forEach.length is defined as 1, so implementation functions would be more native-like if they had their .length property set to 1 too.

http://es5.github.com/#x15.4.4.18

forEach方法的length属性为1.

The length property of the forEach method is 1.

（ func.length 是am func 根据其定义获取的参数的数量。）

(func.length is the amount of argument that func takes based on its definition.)

对于 func.length 要 1 ，你必须定义 func 才能获得1个参数。在函数本身中，您始终可以使用参数获取所有参数。但是，通过定义取1参数的函数， .length 属性为 1 。因此，根据规范更正确。

For func.length to be 1, you have to define func to only take 1 argument. In the function itself you can always get all arguments with arguments. However, by defining the function to take 1 argument, the .length property is 1. Therefore, it is more correct according to the specification.

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