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JavaScript Let

The let keyword is used to declare variables in JavaScript. Variables declared with let are block-scoped, meaning they are only accessible within the block, statement, or expression in which they are defined. Unlike var, let does not allow variables to be redeclared within the same scope.

Key Topics

Block Scope

Variables declared with let are only accessible within the block in which they are defined.

{
    let x = 10;
    console.log(x); // Inside the block
}
console.log(typeof x); // Outside the block

Output

> 10

> undefined

Redeclaring Variables

Variables declared with let cannot be redeclared within the same scope, ensuring a safer programming experience.

let x = 5;
// let x = 10; // Error: Identifier 'x' has already been declared
x = 10; // Allowed: Updating the value
console.log(x);

Output

> 10

Temporal Dead Zone

Variables declared with let are in a "temporal dead zone" from the start of the block until the declaration is encountered. This prevents access before the declaration.

console.log(a); // Error: Cannot access 'a' before initialization
let a = 20;
console.log(a);

Output

> ReferenceError: Cannot access 'a' before initialization

> 20

Examples of Using Let

Below is a practical example of using let in a function and a loop.

function countNumbers() {
    for (let i = 0; i < 3; i++) {
        console.log(i);
    }
    // console.log(i); // Error: i is not defined outside the loop
}
countNumbers();

Output

> 0

> 1

> 2

Key Takeaways

  • Block Scope: let variables are limited to the block in which they are declared.
  • Redeclaration: let prevents redeclaring variables in the same scope.
  • Temporal Dead Zone: Ensures variables cannot be used before they are declared.
  • Best Practice: Use let for variables that may change within their scope.
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