javascript 语法:函数调用和使用括号

问题描述

为什么会这样..

<script type="text/javascript">
<!--

function myAlert(){
    alert('magic!!!');
}


if(document.addEventListener){
    myForm.addEventListener('submit',myAlert,false);
}else{
    myForm.attachEvent('onsubmit',myAlert);
}
// -->
</script>

但不是这个????

<script type="text/javascript">
<!--

function myAlert(){
    alert('magic!!!');
}


if(document.addEventListener){
    myForm.addEventListener('submit',myAlert(),false);
}else{
    myForm.attachEvent('onsubmit',myAlert());
}
// -->
</script>

区别在于调用 myAlert 函数时使用括号.

the difference being the use of parenthesis when calling the myAlert function.

我得到的错误..

"htmlfile: 类型不匹配."VS2008编译时.

推荐答案

函数后面的 () 表示执行函数本身并返回它的值.没有它，您只拥有函数，它可以作为回调传递有用.

The () after a function means to execute the function itself and return it's value. Without it you simply have the function, which can be useful to pass around as a callback.

var f1 = function() { return 1; }; // 'f1' holds the function itself, not the value '1'
var f2 = function() { return 1; }(); // 'f2' holds the value '1' because we're executing it with the parenthesis after the function definition

var a = f1(); // we are now executing the function 'f1' which return value will be assigned to 'a'
var b = f2(); // we are executing 'f2' which is the value 1. We can only execute functions so this won't work

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