javascript - Javascript 提升示例所需的解释

Question

我正在研究一些 Javascript 提升概念并遇到了这个示例。

console.log(a)
var a=5;

function a(){
console.log('In Console')
}

console.log(a)

输出为

function a() {
 console.log('In Console');
} 

5

我不明白两个console.logs的不同行为

谁能解释一下吗？

谢谢!

Answer 1

根据 spec

7. Let declaredFunctionNames be an empty List.

8. Let declaredVarNames be an empty List.

还有

15.b.i Let bindingExists be varEnvRec.HasBinding(vn).

这意味着 JS 引擎将首先提升函数声明，并在迭代变量声明时检查该变量是否已具有绑定(bind)。

解释如下

// Before the following console statement function has been hoisted already and variable declaration is ignored.
console.log(a)

var a=5; // now the value of a is initialized.

//this statement has been hoisted above already
function a(){
console.log('In Console')
}

//value of a is 5 now
console.log(a)