What does js variable promotion mean? Introduction to the use of js variable promotion (example)

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Introduction

Variable hoisting Hoisting is people’s understanding of the way JavaScript execution context works, and it is not an official change

Literally understood, variable promotion means that the declarations of variables and functions will be moved to the front of the scope in the physical layer. But this understanding is not accurate, the effect is the same, but the actual implementation is that the declaration of JavaScript variables and functions will be put into memory during the compilation phase

This means that the user is formally declaring a function or variable We have been able to use it before

Function promotion

In JavaScript, we can use it before declaring a function, everyone should I have experienced it all, like this:

test();

function test() {
    // do something
}

Under normal usage, you should declare the function before calling it, but this method can still run because JavaScript automatically stores the function declaration in memory in advance. The reason is that it looks like JavaScript automatically promotes the function declaration to the front

Variable promotion

For variables, JavaScript uses a similar method, but one thing to note The thing is, for variable promotion, JavaScript will only promote the variable declaration, but will not promote the initialization. If it is used before the variable is initialized, you will get undefined

// undefined
console.log(a);
// ReferenceError: b is not defined
console.log(b);
var a = 10;

// undefined
console.log(num);
num = 6;
// 6
console.log(num);
num += 7;
// 13
console.log(num);
var num;

// undefined
console.log(num);
num = 1;
// 1
console.log(num);
var num = 2;
// 2
console.log(num);

Note here, JavaScript# The variable promotion of ## is for var, while let and const do not have the feature of variable promotion

// ReferenceError: a is not defined
console.log(a);
let a = 10;

A more complicated example

var a = 100;
function fn() {
    // undefined
    console.log(a);
    var a = 200;
    // 200
    console.log(a);
}
fn();
// 100
console.log(a);
var a;
// 100
console.log(a);
// 300
var a = 300;
console.log(a);

Detailed explanation of js variable promotion

In-depth understanding of scope and variable hoisting in JS

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