当使用局部变量将匿名函数传递给命名函数时，Javascript中的范围问题 [英] Scope troubles in Javascript when passing an anonymous function to a named function with a local variable

问题描述

对标题感到抱歉 - 我无法想出一个方法来表达它。

Sorry about the title - I couldn't figure out a way to phrase it.

这是一个场景：

我有一个构建元素的函数：

I have a function that builds a element:

buildSelect(id,cbFunc,...)

在buildSelect内部执行此操作：

Inside buildSelect it does this:

select.attachEvent('onchange',cbFunc);

我还有一个数组：

var xs = ['x1','x2','x3'...];

鉴于所有这些，我有一些代码可以做到这一点：

Given all of these, I have some code that does this:

for(var i = 0; i < xs.length; i++)
{
    buildSelect(blah,function(){ CallBack(xs[i],...) },...);
}

问题是，当onchange被其中一个选项触发时，它正确地去了到CallBack（）但第一个参数不正确。例如，如果我改变第三个选择，我期望用xs [2]调用CallBack（），而不是像xs [3]或其他东西那样得到一些不同的东西。

The issue is that when onchange gets fired on one of those selects it correctly goes to CallBack() but the first parameter is incorrect. For example if I change the third select I expect CallBack() to be called with xs[2] instead I get some varying things like xs[3] or something else.

如果我稍微修改它：

for(var i = 0; i < xs.length; i++)
{
    var xm = xs[i];
    buildSelect(blah,function(){ CallBack(xm,...) },...);
}

我在CallBack（）中的值仍然不正确。有些东西告诉我这是范围/关闭相关但我似乎无法弄清楚是什么。

I'm still getting incorrect values in CallBack(). Something tells me this is scope/closure related but I can't seem to figure out what.

我只想让第一个选择调用CallBack for onchange，第一个参数为xs [0]，第二个选择用xs [1]，依此类推。我在这里可能做错了什么？

I simply want the first select to call CallBack for onchange with the first parameter as xs[0], the second select with xs[1] and so on. What could I be doing wrong here?

我应该澄清xs是一个全局变量。

I should clarify that xs is a global variable.

谢谢

推荐答案

您需要通过在其中关闭它来捕获 xm 值自己的范围。

You need to capture that xm value by closing around it in its own scope.

要做到这一点，需要单独的函数调用：

To do this requires a separate function call:

buildCallback( curr_xm ) {

      // this function will refer to the `xm` member passed in
    return function(){ CallBack(curr_xm,...) },...);
}

for(var i = 0; i < xs.length; i++)
{
    var xm = xs[ i ];
    buildSelect(blah,buildCallback( xm ),...);
}

现在 xm 回调引用的是您传递给 buildCallback 的那个。

Now the xm that the callback refers to is the one that you passed to buildCallback.

如果你还需要保留 i 的其他用途，你可以发送它：

If you have other uses for i that need to be retained, you could send that instead:

buildCallback( curr_i ) {


      // this function will refer to the `i` value passed in
    return function(){ CallBack( xs[ curr_i ],...) },...);
}

for(var i = 0; i < xs.length; i++)
{
    buildSelect(blah,buildCallback( i ),...);
}

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