函数调用和函数引用有什么区别？ [英] What is the difference between a function call and function reference?

问题描述

我有以下功能：

$ p $ function hello（）{
alert（hi！）;
}


取这段代码：

  var elem = document.getElementById（btn）; 
 elem.onclick = hello; 
  

我的问题可能有点难理解，请耐心等待：
什么是正确的将这段代码与普通调用区分开来，还是让这段代码需要引用函数变量而不是常规调用？ （ hello（）; ）

如何知道我应该在哪里给出函数的引用，当我应该实际调用它的时候？

解决方案

那么， onclick 属性期望引用一个函数，以便在元素被点击时执行。通常它可以是：

  element.onclick = funcRef; 
  

或

  element.onclick = function（）{
 funcRef（）; 
}; 
  

（但当然，最好使用 addEventListener 和 attachEvent ）

注意它们都是函数的引用，而不是调用。

当某些事情需要引用时，你不会调用它...你为它分配一个引用（第一个例子）。

当你想专门调用一个函数时，可以用（）来调用它（第二个例子）。但请注意，在第二个示例中，仍然有一个对分配给 onclick 的函数的引用 - 它只是一个匿名函数。

可能是更重要的部分：

有些人认为您想这样做：

  element.onclick = funcRef（）; 
  

但是这会立即执行该函数（因为（）），并将其返回值分配给 onclick 。除非返回值是函数，否则这不是你想要的。

我认为这个故事的寓意是，当你想要/需要一些东西来执行，你可以调用这个函数。如果该功能需要以后使用或需要存储，则不要调用它。

I have the following function

function hello() {
 alert("hi!");
}

Take this piece of code:

var elem = document.getElementById("btn");
elem.onclick = hello;

My question might be a bit hard to understand, so bear with me: What EXACTLY differentiates THIS piece of code from a normal call, or what makes this piece of code require a reference to the function variable rather than a regular call? (hello();)

How can I know where I'm supposed to give a reference to the function, and when I'm supposed to actually call it?

解决方案

Well, the onclick property expects a reference to a function, for it to execute when the element is clicked. Normally it's either:

element.onclick = funcRef;

or

element.onclick = function () {
    funcRef();
};

(but of course, it's best to use addEventListener and attachEvent)

Notice how both of them are references to functions, not calling.

When something expects a reference, you don't call it...you assign a reference to it (first example).

When you want to specifically call a function, you call it with () (second example). But notice how in the second example, there's still a reference to a function that's assigned to onclick - it's just an anonymous function.

Probably the more important part:

Some people think you want to do this:

element.onclick = funcRef();

But that immediately executes the function (because of the ()), and assigns its return value to onclick. Unless the return value is a function, this isn't what you want.

I think the moral of the story is that when you want/need something to execute right now, you call the function. If the function is wanted for later use or needs stored, you don't call it.

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