Java語言吊裝示例所需的說明

Question

我正在研究一些Javascript提升概念，並遇到了這個示例。

console.log(a)
var a=5;

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

console.log(a)

輸出是

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

5

我不了解兩個console.logs的不同行為

誰能解釋？

謝謝！

Answer 1

根據規格

7. 讓clarifiedFunctionNames為空列表。

8. 令clarifiedVarNames為空列表。

也

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

這意味着JS引擎將首先提升功能聲明，並在遍歷變量聲明的同時檢查該變量是否已具有該名稱的綁定。

解釋如下

// 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

隨其主體懸掛的函數聲明（按規范）。 因此，請在首次log之前將function a吊起。 然后，重新分配變量a ，第二個log 5。

因此，提升后，您的功能將變為以下內容：

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)