The difference between let and var in js

The main difference between let and var declared variables is scope: variables declared with let are only visible within the block-level scope where they are declared, while variables declared with var have global or function scope. In addition, let does not allow redeclaration, but allows reassignment; var allows redeclaration and assignment.

The difference between let and var

let and var are two keywords for declaring variables in JavaScript. There are the following key differences in scoping, hoisting, and redeclaration:

Scope

• var: A variable declared by var has Global scope or function scope, depending on where it is declared.
• let: Variables declared by let are only visible within the block-level scope (curly braces) where they are declared.

Promotion

• var: Variables declared with var will be promoted to the function or global scope before executing the code the top of.
• let: Variables declared with let are not promoted, they are only available within the scope in which they are declared.

Redeclaration

• var: Variables declared with var can be redeclared in the same scope and will be overwritten Previous Statement.
• let: Variables declared with let cannot be redeclared in the same scope, which will throw an error.

Other differences

In addition to the above main differences, let and var have the following other differences:

• Reassignment: Variables declared with let can be reassigned, while variables declared with var can be reassigned and redeclared.
• Initial value: Variables declared by let must be initialized when declared, while variables declared by var may not be initialized and default to undefined.

Example

// 使用 var
function example1() {
  var x = 10;
  if (true) {
    var x = 20; // 重新声明和重新赋值
    console.log(x); // 输出: 20
  }
  console.log(x); // 输出: 20
}

// 使用 let
function example2() {
  let y = 10;
  if (true) {
    let y = 20; // 重新赋值
    console.log(y); // 输出: 20
  }
  console.log(y); // 输出: 10
}

As shown in the example, var allows redeclaration and reassignment of variables in the same scope, while let only allows reassignment.

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