如何在Javascript .filter（）方法中将额外参数传递给回调函数？ [英] How do I pass an extra parameter to the callback function in Javascript .filter() method?

问题描述

我想将Array中的每个字符串与给定的字符串进行比较。我目前的实现是：

I want to compare each string in an Array with a given string. My current implementation is:

function startsWith(element) {
    return element.indexOf(wordToCompare) === 0;
}
addressBook.filter(startsWith);

这个简单的功能有效，但只是因为现在正在设置 wordToCompare 作为一个全局变量，但我当然希望避免这种情况并将其作为参数传递。我的问题是我不知道如何定义 startsWith（）因此它接受一个额外的参数，因为我真的不明白它所采用的默认参数是如何传递的。我已经尝试了所有我能想到的不同方法，但没有一种方法可以工作。

This simple function works, but only because right now wordToCompare is being set as a global variable, but of course I want to avoid this and pass it as a parameter. My problem is that I am not sure how to define startsWith() so it accepts one extra parameter, because I dont really understand how the default parameters it takes are passed. I've tried all the different ways I can think of and none of them work.

如果你还可以解释传递参数如何内置回调函数（对不起，我不知道这些更好的术语）工作会很棒

If you could also explain how the passed parameters to 'built in' callback functions (sorry, I dont know of a better term for these) work that would be great

推荐答案

制作 startsWith 接受要比较的单词和返回一个函数，然后将其用作过滤/回调函数：

Make startsWith accept the word to compare against and return a function which will then be used as filter/callback function:

function startsWith(wordToCompare) {
    return function(element) {
        return element.indexOf(wordToCompare) === 0;
    }
}

addressBook.filter(startsWith(wordToCompare));

另一种选择是使用 Function.prototype.bind ^[MDN] （仅在支持ECMAScript 5的浏览器中可用，请按照旧版浏览器的垫片链接）并修复第一个参数：

Another option would be to use Function.prototype.bind ^[MDN] (only available in browser supporting ECMAScript 5, follow a link for a shim for older browsers) and "fix" the first argument:

function startsWith(wordToCompare, element) {
    return element.indexOf(wordToCompare) === 0;
}

addressBook.filter(startsWith.bind(this, wordToCompare));

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我真的不是了解如何传递默认参数

I dont really understand how the default parameters it takes are passed

没有什么特别之处。在某些时候， filter 只是调用回调并传递数组的当前元素。所以它是一个调用另一个函数的函数，在这种情况下是你作为参数传递的回调。

There is nothing special about it. At some point, filter just calls the callback and passes the current element of the array. So it's a function calling another function, in this case the callback you pass as argument.

这是一个类似函数的例子：

Here is an example of a similar function:

function filter(array, callback) {
    var result = [];
    for(var i = 0, l = array.length; i < l; i++) {
        if(callback(array[i])) {  // here callback is called with the current element
            result.push(array[i]);
        }
    }
    return result;
}

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