What are the uses of let and const in js

Let and const are used to declare variables in JavaScript. The main difference is scope, variability and initialization time: Scope: let is limited to block-level scope, while const is valid in block-level and function scope. Mutability: mutable if declared by let, immutable if declared by const. Initialization: let can be assigned at the time of declaration or later, const must be assigned at the time of declaration and can only be assigned once.

Usage of let and const in JavaScript

In JavaScript, let and const keyword is used to declare variables. They all have different behaviors in terms of scope, mutability, and initialization time.

let

• Scope: let The declared variable is only valid within the block-level scope where it is declared.
• Variability: let The declared variable is variable, that is, it can be reassigned.
• Initialization: let A declared variable can be initialized immediately when declared, or it can be initialized later by assignment. If not initialized, its value is undefined.

const

• Scope: const The declared variable is within the block-level scope and function scope where it is declared. efficient.
• Variability: const The declared variable is immutable, that is, it cannot be reassigned.
• Initialization: const The declared variable must be initialized immediately when declared and can only be assigned a value once.

When to use?

• Use let to declare mutable variables in the local scope.
• Use const to declare constant or immutable variables.
• Use const to avoid inadvertently modifying variables.
• Use let to declare variables that may be reassigned after declaration.

Example

// 使用 let 声明一个可变变量
let count = 0;

// 使用 const 声明一个常量
const PI = 3.14;

// 重新赋值 let 声明的变量
count = 1;

// 尝试重新赋值 const 声明的变量会报错
PI = 3.14159; // TypeError: Assignment to constant variable.

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