使用for循环设置事件处理程序 [英] Setting event handlers using a for loop

问题描述

我搞乱了一些javascript，在jsfiddle，并遇到一个奇怪的问题。我似乎无法弄清楚为什么我无法通过for循环设置 onclick 事件处理程序：

I was messing around with some javascript, on jsfiddle, and ran into a strange issue. I can't seem to figure out why I am unable to set onclick event handlers via a for loop:

html：

<table border="1" cellspacing="1" width="500">
    <tr id="headerRow">
        <td>Header1</td>
        <td>Header2</td>
        <td>Header3</td>
        <td>Header4</td>
    </tr>
    <tr>
        <td>books</td>
        <td>red</td>
        <td>peas</td>
        <td>321</td>
    </tr>
    <tr>
        <td>tapes</td>
        <td>blue</td>
        <td>jello</td>
        <td>654</td>
    </tr>
</table>

在DOM中执行的js：

js executed in DOM ready:

var arr = document.getElementById('headerRow')
    .getElementsByTagName("td");

// Why does this work??
/*arr[0].onclick = function() { alert(arr[0].innerHTML); };
arr[1].onclick = function() { alert(arr[1].innerHTML); };
arr[2].onclick = function() { alert(arr[2].innerHTML); };
arr[3].onclick = function() { alert(arr[3].innerHTML); };
*/

//But this doesn't????
for(var i = 0; i < arr.length; i++) {
    arr[i].onclick = function() { alert(arr[i].innerHTML); };
}

http://jsfiddle.net/xzmMj/4/

推荐答案

i 不会包含你想要的当前索引，而是最后一个值 i c $ c> arr.length

i will not contain the "current index" like you are intending, but rather the last value i was, ie arr.length

一个快速肮脏的解决方案是做这样的事

One quick n dirty solution would be to do something like this

for(var i = 0; i < arr.length; i++) {
    (function(_i){
        arr[_i].onclick = function() { alert(arr[_i].innerHTML); };
    })(i);
}

这样做是为了捕获 i的当前值在您正在执行的语句的闭包内的一个新变量 _i 中，因此它将留在周围并成为您期望的值每次调用onclick处理程序。

what this does is captures the current value of i in a new variable _i within the closure of the statement you're executing, so it'll stay around and be the value you'd expect every time your onclick handler is called.

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