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=43. 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=3For more programming-related knowledge, please visit: Introduction to Programming! !
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