Javascript - 悬挂优先 [英] Javascript - Precedence in hoisting

问题描述

在提升中，变量优先于函数定义还是相反？请参阅下面的代码：

In hoisting, do variables take precedence over function definition or the other way round? Please see the code below:

function a()
{
    var x = 10;

    function x() {
        return 20;
    }

    return x;
}

推荐答案

这不是一个问题优先于另一个（有优先级发生，但这主要是一个语义问题）。

It's not a matter of one taking precedence over the other (there is precedence taking place, but that's largely just a matter of semantics).

重要的是变量声明的赋值部分是 not ，而整个函数定义是 >。

What matters here is that the assignment part of the variable declaration is not hoisted, whereas the entire function definition is.

注意根据raina77ow的回答，似乎我原来的部分假设是错误的。函数在变量声明前被挂起，但净效果是相同的。

Note Based on raina77ow's answer, it seems that part of my original assumptions were wrong. Functions are hoisted before variable declarations, but the net effect is the same.

提升后，您的函数将如下所示：

After hoisting, your function will act like this:

function a()
{
    var x = function x() {  // hoisted function declaration/definition
        return 20;
    };
    var x;                  // hoisted variable declaration
    x = 10;                 // unhoisted part of variable declaration
    return x;
}

x = 10 在完成所有提升后发生，因此这是 x 中的值。

---

为了回应@ thefourtheye的请求（我认为这是他/她所要求的），如果您的原始函数如下所示：

the x = 10 takes place after all the hoisting is done, so that is the value that remains in x.

---

To respond to @thefourtheye's request (I think this is what s/he is asking for), if your original function looked like this:

function a() {
    function x() {
        return 20;
    }
    var x = 10;
    return x;
}

然后在看起来像这样（与上面相同）：

Then after hoisting, it would look like this (same as above):

function a() {
    var x = function x() {  // hoisted function declaration/definition
        return 20;
    }
    var x;                  // hoisted variable declaration (does nothing)
    x = 10;                 // unhoisted variable assignment
    return x;
}

---

，试试这个：

---

As one last illustration, try this:

function a() {
    console.log(x);
    var x = 10;
    console.log(x);
    function x() { return 20; };
}

打印出来时：

When called, this prints out:

function x() { return 20; }
10

原因在于提升会导致函数的行为像这样：

The reason for this is that hoisting is causing the function to behave like this:

function a() {
    var x = function x() { return 20; };
    var x;
    console.log(x);
    x = 10;
    console.log(x);
}

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