返回函数的函数 [英] Functions that return a function

问题描述

我坚持使用'返回函数的函数'这个概念。我指的是Stoyan Stefanov撰写的面向对象的Javascript一书。

I'm stuck with this concept of 'Functions that return functions'. I'm referring the book 'Object Oriented Javascript' by Stoyan Stefanov.

Snippet One：

    function a() {
      
        alert('A!');
    
        function b(){
            alert('B!'); 
        }
    
        return b();
    }
    
    var s = a();
    alert('break');
    s();

输出

A!
B!
break

Snippet Two

function a() {
  
    alert('A!');

    function b(){
        alert('B!'); 
    }

    return b;
}

var s = a();
alert('break');
s();

输出：

A!
break
B!

有人可以告诉我返回 b 和 b（）在上面的片段中？

Can someone please tell me the difference between returning b and b() in the above snippets?

推荐答案

分配函数的变量（没有括号）复制对函数的引用。将括号放在函数名的末尾，调用函数，返回函数返回值。

Assigning a variable to a function (without the parenthesis) copies the reference to the function. Putting the parenthesis at the end of a function name, calls the function, returning the functions return value.

function a() {
    alert('A');
}
//alerts 'A', returns undefined

function b() {
    alert('B');
    return a;
}
//alerts 'B', returns function a

function c() {
    alert('C');
    return a();
}
//alerts 'C', alerts 'A', returns undefined

alert("Function 'a' returns " + a());
alert("Function 'b' returns " + b());
alert("Function 'c' returns " + c());

在您的示例中，您还在函数中定义函数。如：

In your example, you are also defining functions within a function. Such as:

function d() {
    function e() {
        alert('E');
    }
    return e;
}
d()();
//alerts 'E'

该功能仍可调用。它仍然存在。这在JavaScript中一直使用。函数可以像其他值一样在 周围传递。请考虑以下事项：

The function is still callable. It still exists. This is used in JavaScript all the time. Functions can be passed around just like other values. Consider the following:

function counter() {
    var count = 0;
    return function() {
        alert(count++);
    }
}
var count = counter();
count();
count();
count();

函数计数可以保留在其外部定义的变量。这称为闭包。它在JavaScript中也经常使用。

The function count can keep the variables that were defined outside of it. This is called a closure. It's also used a lot in JavaScript.

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