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Look at the initialization method from the duplicate names of JavaScript functions_javascript tips

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Release: 2016-05-16 19:17:45
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今天有个哥们问我要是JavaScript函数重名了会有什么后果?开始我没有细想,就说可能会出错吧,可是等我实验完了发现页面没有任何脚本错误提示,而且程序也运行了,只是对同名函数的调用执行了位置靠后的一个。

    回头仔细一想,这个结果完全可以接受,因为脚本在页面里本身就是顺序执行下来的,包括函数的定义,但然如果只是定义 function foo(){} 这种形式,我们是跟踪不到函数初始化的。可是如果是定义类的方式,我们就可以明显地跟踪到函数的初始化顺序。比如:

Look at the initialization method from the duplicate names of JavaScript functions_javascript tipsfunction foo() {}
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips
function foo.prototype.fn1() {}
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips
function foo.prototype.fn2() {}

    我们就可以明显地看到先执行function foo.prototype.fn1(){}再执行function foo.prototype.fn2(){}。

    回到我们刚才说的JavaScript脚本函数重名问题上来,比如我们定义两个函数 funAlert():

Look at the initialization method from the duplicate names of JavaScript functions_javascript tipsfunction funAlert()
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips{
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips    alert('A');
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips}
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips
function funAlert()
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips{
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips    alert('B');
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips}

    调用 funAlert(),那么将显示一个MegBox,内容为'B'。

    为什么初始化函数会有这样的效果?这里只用把上面两个函数的定义改一下,就会一目了然了,我们把定义改为:

Look at the initialization method from the duplicate names of JavaScript functions_javascript tipsvar fnAlert = new Function("alert('A')");
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips
var fnAlert = new Function("alert('B')");
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips
Look at the initialization method from the duplicate names of JavaScript functions_javascript tipswindow.fnAlert();
    其函数也就是定义在对象上的一个函数指针,我们象这个指针赋什么函数引用,它就执行什么效果,而JavaScript中的脚本函数重名就和普通赋值语句一样,等同于:
Look at the initialization method from the duplicate names of JavaScript functions_javascript tipsvar i = 0;
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips
var i = 1;
    // 稍微注意以下JavaScript里的var,用var定义变量和我们平时用的高级语言定义变量是不同的,它只起到一个提示的作用,提醒说我在这里定义变量了,而没有什么变量作用域的概念,只要不离开定义它的对象的域(比如页面刷新),出现过的变量会就一直存在。所以var写不写都行。举个例子:
Look at the initialization method from the duplicate names of JavaScript functions_javascript tipsif ( true )
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips{
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips   t 
= 100;
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips}
Look at the initialization method from the duplicate names of JavaScript functions_javascript tipsalert(t);

will display 100, while

Look at the initialization method from the duplicate names of JavaScript functions_javascript tipsif ( true )
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips{
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips
var t = 100;
Look at the initialization method from the duplicate names of JavaScript functions_javascript tips}
Look at the initialization method from the duplicate names of JavaScript functions_javascript tipsalert( t);

It also displays 100.

So whether JavaScript script function names are repeated is just a matter of operation , which is completely different from the grammatical constraints in our high-level languages, and of course it is not the category of overload.

What is the use of having the same script function name? The most intuitive thing is that it can be used to implement pseudo-overloading . For example, many of our free homepage spaces often impose pop-up window advertisements on you. We can write:

on the first line of the page.
<script language="javascript">
 var _open = window.open;
 window.open 
= function() {}
script >
This way you can block pop-up window ads that are not added to the first line of the page (adding to the first line cannot be blocked because window.open It is executed before it is 'overloaded' open).
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