在JavaScript中调用带有括号和不带有括号的函数的区别 [英] Difference of calling a function with and without parentheses in JavaScript

问题描述

我正在处理一个JavaScript文件上传事件.而且我有以下初始化程序和以下函数:

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

初始化器

    $('#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，我发现如果这样做，则有所不同

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

before_add:progressBar.show 版本 before_add:progressBar.show().有括号的话，即使绑定到 before_add 选项，它也会被调用一次，而没有括号的话，它将不会被调用.

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

我观察到的行为是否有解释?

Is there an explanation for the behaviour I observed?

推荐答案

使用括号，因为括号和结果被调用该调用的em>将存储在before_add中.

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

不包含的括号中，您存储了对该变量中函数的引用(如果可以，则为指针").这样，只要有人调用before_add()，它就会被调用.

Without the parentheses 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().

如果这不能解决问题，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** parentheses:
var z = Bar('Whut?');

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

如果您随后查看浏览器的控制台窗口，应该会看到:

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() 通过" y ，
第3行是 y ，
的内容"第4行是 var z = Bar('Whut?'); 行的结果；Bar功能被调用，
第5行显示调用 Bar()并将结果分配给 z 不会返回任何内容(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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