The difference between const, var and let in js

The main content is: the differences between the three ways of defining variables in js: const, var, and let.

1. Variables defined by const cannot be modified and must be initialized.

const b = 2; // Correct

// const b; // Error, must be initialized

console.log('Const definition b outside function: ' + b); // There is an output value

// b = 5;

// console.log('Modify const definition b outside function: ' + b); //Cannot output

2. 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 is a block-level scope. After the function is defined using let, 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