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HomeWeb Front-endJS TutorialThe difference between let and var in js

The main differences between let and var in JavaScript are as follows: Scope: let is only visible within the declaration block, while var is visible within the entire function or global scope. Declaration hoisting: var declarations are promoted to the top of the scope, let is only available at the declaration position. Redeclaration: redeclaration of let will generate an error, while var will overwrite its value.

The difference between let and var in js

The difference between let and var in JavaScript

let and ## in JavaScript #var are two different ways of declaring variables, with key differences in scope, declaration hoisting and redeclaration.

Scope

  • var: var Variables declared in a function or global scope are used throughout Functions or global scopes are visible even if they are nested within other blocks of code.
  • let: Variables declared in block-level scope (such as if statements, for loops, or functions) let are only visible within that block.

Declaration hoisting

  • var: Before the code is executed, variables declared by var will Promoted to the top of the scope. This creates uninitialized variables and may cause unexpected behavior.
  • let: Not promoted before code execution, only available where the variable is declared.

Redeclaration

  • var: If var variable is redeclared within the same scope, Its value will be overwritten.
  • let: Redeclaring a let variable within the same scope will cause a syntax error because it already exists.

Example

// var 声明全局变量,即使在函数内也可见
var x = 10;

// let 声明块级变量,仅在 if 语句中可见
if (true) {
  let y = 20;
  console.log(y); // 20
}

console.log(x); // 10
// console.log(y); // 错误:y 未定义

Other differences

  • let and const are all block-level scope variables, but const represents a constant value and cannot be reassigned.
  • var is more common in older JavaScript code, but let and const are recommended as it provides cleaner and safer code.

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