Java语言吊装示例所需的说明

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)

输出是

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

5

我不了解两个console.logs的不同行为

谁能解释？

谢谢！

Answer 1

根据规格

7. 让clarifiedFunctionNames为空列表。

8. 令clarifiedVarNames为空列表。

也

15.bi令bindingExists为varEnvRec.HasBinding（vn）。

这意味着JS引擎将首先提升功能声明，并在遍历变量声明的同时检查该变量是否已具有该名称的绑定。

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)

Answer 2

随其主体悬挂的函数声明（按规范）。 因此，请在首次log之前将function a吊起。 然后，重新分配变量a ，第二个log 5。

因此，提升后，您的功能将变为以下内容：

var a;

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

console.log(a)
a=5;
console.log(a)

Answer 3

// at this point a is a function, variable below has not yet been assigned. 
// functions can be called from anywhere in the code file (not taking scope into account)
console.log(a)
// a is now assigned to a variable and assigned to 5, it no longer refers to the function
var a=5;

// function defined as a which can be called as a until a is reassigned
function a(){
console.log('In Console')
}

// a now refers to 5 at this point in the execution
console.log(a)