JavaScript Let

JavaScript let – Block Scope Variable Declaration

In JavaScript, let is a keyword used to declare variables. It was introduced in ES6 (ECMAScript 2015) and offers better scoping and flexibility compared to the older var keyword.

Key Features of let:

1. Block Scope

Variables declared with let are block-scoped, meaning they are only accessible within the block {} where they are defined.

Example:

{
  let x = 10;
  console.log(x); // 10
}
// console.log(x); // Error: x is not defined

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2. No Hoisting (Initialization in Temporal Dead Zone)

Although variables declared with let are hoisted, they are not initialized until their definition is encountered. This results in a “Temporal Dead Zone” from the start of the block until the variable is declared.

Example:

// console.log(y); // Error: Cannot access 'y' before initialization
let y = 5;
console.log(y); // 5

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3. No Re-declaration in the Same Scope

Unlike var, let does not allow the same variable to be declared more than once in the same scope.

Example:

let a = 10;
// let a = 20; // Error: Identifier 'a' has already been declared

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4. Can Be Updated

Variables declared with let can be updated within their scope.

Example:

let count = 1;
count = 2; // Updating the variable
console.log(count); // 2

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5. Use Cases

  • Use let when you expect the value of the variable to change.
  • Suitable for loops and conditions where variables change over time.

Example:

for (let i = 0; i < 5; i++) {
  console.log(i);
}

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Summary

  • let is block-scoped, preventing unintended access outside the block.
  • It avoids issues related to hoisting seen with var.
  • It provides better control over variable re-declaration, making code more robust and easier to debug.

Using let is generally preferred over var for variable declarations in modern JavaScript development.