What is the difference between var, let and const in JavaScript?

Difference: 1. The var variable is a function or global scope, and the let and const variables are the block scope; 2. The let and const variables are subject to the temporary dead zone, but the var variable is not; 3. var Variables can be allocated and declared, letr variables can be allocated and cannot be redeclared, and const variables cannot be reallocated and declared.

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We all know that in the latest ES6, a new There are two keywords let and const used for variable declaration. So what is the difference between these two and the var we often use before? Let’s take a look today.

1. const statement

The const statement is used to declare a variable in JavaScript. The variable follows the following rules:

• belongs to the block scope of.

• is subject to the "temporary dead zone".

• It does not create any global properties on window.

• Not reassignable.

• Cannot be redeclared.

The variables defined by the const statement cannot be modified and must be initialized.

const b = 2;//正确
// const b;//错误，必须初始化 
console.log('函数外const定义b：' + b);//有输出值
// b = 5;
// console.log('函数外修改const定义b：' + b);//无法输出

2. var statement

The var statement is used to declare a variable in JavaScript. The variable follows the following rules:

• The scope is Function scope or global scope.

• is not restricted by the temporary dead zone (TDZ).

• It will create a global property on window with the same name.

• is reassignable.

• is redeclarable.

Variables defined by var can be modified. If not initialized, undefined will be output and no error will be reported.

var a = 1;
// var a;//不会报错
console.log('函数外var定义a：' + a);//可以输出a=1
function change(){
a = 4;
console.log('函数内var定义a：' + a);//可以输出a=4
}
change();
console.log('函数调用后var定义a为函数内部修改值：' + a);//可以输出a=4

3. let statement

let statement declares a variable in JavaScript, which follows the following rules:

• Belongs to block scope.

• is subject to the temporary dead zone.

• It does not create any global properties on window.

• is allocatable.

• Cannot be redeclared.

let belongs to the block-level scope. After the let is defined inside the function, it has no impact on the outside of the function.

let c = 3;
console.log('函数外let定义c：' + c);//输出c=3
function change(){
let c = 6;
console.log('函数内let定义c：' + c);//输出c=6
}
change();
console.log('函数调用后let定义c不受函数内部定义影响：' + c);//输出c=3

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