JavaScript吊装说明 [英] JavaScript hoisting explanation

问题描述

以下摘要之间有什么区别?

What is the difference between below snippets?

var a = 0;
function b(){
    a = 10;
    return function a(){};
}
b();
console.log(a);  // => 10

和

var a = 0;
function b(){
    a = 10;
    return
    function a(){};
}
b();
console.log(a);  // => 0

这与JavaScript提升有关，但是我对该概念的理解却给出了完全相反的输出.

It has something to do with JavaScript hoisting, but my understanding of the concept gives exactly the opposite output.

推荐答案

return function a(){};

此处function ...是一个表达式.准确地说，是命名函数表达式.此处的a没什么大不了，它只是为匿名函数提供了

Here function ... is an expression. A named function expression to be precise. The a here doesn't matter much, it just gives the anonymous function a .name, but it's still just a function expression that you're returning.

return 
function a(){};

这等效于:

return;
function a(){};

此处function a是声明，而不是表达式.它被吊起，在作用域中创建本地名称a，遮盖了外部a. IE.它等效于:

Here function a is a declaration, not an expression. It is hoisted, creating a local name a in the scope, shadowing the outer a. I.e. it is equivalent to:

function b(){
    var a = function () {};
    a = 10;
    return;
}

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