Variable function declaration priority in Javascript

Case One

console.log(a); //输出function a(){}
function a() {};
var a=3;

Case Two

function a() {};
var a=3;
console.log(a);//输出3

Why are the outputs of the two different?

Analysis:

For case one

The execution process can be refined as

#1. First, it will apply for a space, start to declare function a, and then declare variable a during execution , it is found that there is already a declaration of a, so no declaration is made

2. Then, console.log(a) is executed; that is, the output function a

#3. The final execution is to perform the assignment operation, and set a=3. At this time, the function a is converted to a variable type and assigned a value of 3. For verification, you can change the program to the following

#

a();
function a() {
	console.log(5);
};
var a=3;
a();

You can find that the program execution results are as follows:

##Because before the assignment, a is still a function. After the assignment statement is executed, a It becomes 3, and an error will be reported when executing a().

For Case 2

# #Similarly, it can be seen that function a is first declared, then declares variable a, and finds that there is already a declaration of a, so it does not declare it. Next, the assignment statement is executed to convert function a into a variable. Finally output the value of variable a

Summary:

1)函数被申明的优先级高于变量

2)被声明过的变量名或是函数名不会被重复申明

3)变量和赋值语句一起书写，在js引擎解析时，会将其拆成声明和赋值2部分，声明置顶，赋值保留在原来位置

补充

1.下面代码执行结果为

var x=0;
f();
console.log(x);
var f=function(){
	x=1;
}
f();
console.log(x);
function f(){
	x=2;
}
f();
console.log(x);

答案：2 1 1 

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