为什么Javascript函数在实例化时的行为与执行时不同? [英] Why does a Javascript function act differently upon instantiation than execution?

问题描述

我来自C#/PHP，试图弄清Javascript的观点，即函数是变量/对象，并且具有类似的构造函数，等等.

I'm coming from C#/PHP and trying to get my head around Javascript's idea that functions are variables/objects and have quasi constructors, etc.

任何人都可以解释以下代码为何起作用，即:

Can anyone explain why the following code functions as it does, namely:

1. 为什么实例化变量/函数test时不显示"2"?
2. 为什么执行变量/功能test时不显示"1"?

1. Why isn't "2" displayed when instantiating the variable/function test?
2. Why isn't "1" displayed when executing the variable/function test?

代码:

var setup = function () {
    console.log(1);
    return function() {
        console.log(2);
    };
};

var test = setup(); // 1
test(); // 2
test(); // 2
test(); // 2

已添加:

感谢@thejh @Justin，因此该函数返回的完全不同的函数与第一个函数无关(我当时认为第二个函数是第一个函数的一种构造函数)，如果我将其注释掉，那就更清楚了:

added:

Thanks @thejh @Justin so the function is returning a completely different function that has nothing to do with the first (I was thinking of the second function as a kind of constructor of the first), if I comment it out, it's clearer:

$(document).ready(function() {

    var setup = function () {
        console.log(1);
//        return function() {
//            console.log(2);
//        };
    };

    var test = setup(); // 1
    test(); // "test is not a function"
    test(); // "test is not a function"
    test(); // "test is not a function"

});

推荐答案

您仅在第一次呼叫setup().一旦调用，它将返回的新函数分配给test.从那里开始，您要调用该新函数:

You're only calling setup() the first time. Once it is called, the new function it returns is assigned to test. From there on, you're calling that new function:

// calls setup which logs 1 and returns a new function.
// setup also returns a new function and assigns that new function to test.
var test = setup(); 

// test now is the equivalent of var test = function(){ console.log(2); };

// call the new function that setup returned which logs 2
test();

// and again
test();

// and again
test();

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