Javascript中有和没有括号的调用函数的区别 [英] Difference of calling function with and without parenthesis in Javascript

问题描述

我正在处理javascript文件上传事件。我有以下功能和初始化程序：

I was handling a javascript file upload event. and I have the following function and following initializer:

初始化程序

    $('#s3-uploader').S3Uploader({
        allow_multiple_files: false,
        before_add: progressBar.show,
        progress_bar_target: $('.upload-progress-bar'),
        remove_completed_progress_bar: false
    }).bind("s3_upload_complete", function(e, content) {
        console.log(content);
    });

功能

var progressBar = {
    show: function() {
        $('.upload-progress-bar').show();
        return true;
    }
}

在初始化程序中，我注意到如果有差异，我

In the initializer, I noticed there is a difference if I do

before_add：progressBar.show vs before_add：progressBar.show（）。使用括号，即使它被绑定到 before_add 选项，它也会被调用一次，而没有括号则不会。

before_add: progressBar.show v.s. before_add: progressBar.show(). With the parenthesis, it will be called once even if it is bound to the before_add option, and without the parenthesis it will not.

我想知道我观察到的行为是否有任何解释。

I wonder if there are any explanation to the behaviour I observed.

推荐答案

使用括号调用方法因为括号的，该调用的结果将存储在before_add中。

With parenthesis the method is invoked because of the parenthesis, the result of that invocation will be stored in before_add.

如果没有括号，则将引用（或指针）存储到变量中的函数中。这样，只要有人调用before_add（），就会调用它。

Without the parenthesis you store a reference (or "pointer" if you will) to the function in the variable. That way it will be invoked whenever someone invokes before_add().

如果那不清楚的话;也许这会有所帮助：

If that didn't clear things up; maybe this will help:

function Foo() {
    return 'Cool!';
}

function Bar(arg) {
    console.log(arg);
}

// Store the >>result of the invocation of the Foo function<< into X
var x = Foo();
console.log(x);

// Store >>a reference to the Bar function<< in y
var y = Bar;
// Invoke the referenced method
y('Woah!');

// Also, show what y is:
console.log(y);

//Now, try Bar **with** parenthesis:
var z = Bar('Whut?');

//By now, 'Whut?' as already been output to the console; the below line will
//return undefined because the invokation of Bar() didn't return anything.
console.log(z);

如果您再看一下浏览器的控制台窗口，您应该看到：

If you then take a look at your browsers' console window you should see:

Cool!
Woah!
function Bar(arg)
Whut?
undefined

第1行是调用 Foo（）的结果，

第2行是调用 Bar（） via <$的结果c $ c> y ，

第3行是 y 的内容，

第4行是 var z = Bar（'Whut？'）; 行的结果; Bar函数调用，

第5行显示调用 Bar（）并将结果赋值给 z 没有返回任何内容（因此： undefined ）。

Line 1 is the result of invoking Foo(),
Line 2 is the result of invoking Bar() "via" y,
Line 3 is the "contents" of y,
Line 4 is the result of the var z = Bar('Whut?'); line; the Bar function is invoked,
Line 5 shows that invoking Bar() and assigning the result to z didn't return anything (thus: undefined).

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