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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);Related recommendations:
Detailed explanation of js variable promotion
In-depth understanding of scope and variable hoisting in JS
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