let The let keyword in JavaScript is used to declare variables with block scope. It was introduced in ES6 (ECMAScript 2015) as an improvement over var, which had function scope and caused issues in larger programs.
letTo declare a variable using let, the syntax is:
let variableName = value;
Unlike const, let variables can be updated, but they cannot be redeclared in the same scope.
let user = "Alice";
console.log(user); // Output: Alice
You can change the value of a let variable:
let age = 25;
age = 30; // ✅ Allowed (Updating the value)
console.log(age); // Output: 30
However, you cannot redeclare the same variable in the same scope:
let age = 25;
let age = 30; // ❌ Error! 'age' has already been declared
let is Block-ScopedA block in JavaScript refers to any section of code enclosed in {}.
let exist only within the block where they are defined.if (true) {
let message = "Hello, World!";
console.log(message); // ✅ Works inside the block
}
console.log(message); // ❌ Error! 'message' is not defined outside the block
message was declared inside the {} block, it does not exist outside it.for (let i = 0; i < 3; i++) {
console.log(i); // ✅ Works inside the loop
}
console.log(i); // ❌ Error! 'i' is not accessible outside the loop
let Allows Reassignment But Not Redeclarationlet city = "New York";
city = "Los Angeles"; // ✅ No error, value is updated
console.log(city); // Output: Los Angeles
let city = "New York";
let city = "Chicago"; // ❌ Error! 'city' has already been declared
let vs var vs const| Feature | let |
var |
const |
|---|---|---|---|
| Scope | Block-scoped | Function-scoped | Block-scoped |
| Can be Reassigned? | ✅ Yes | ✅ Yes | ❌ No |
| Can be Redeclared in the Same Scope? | ❌ No | ✅ Yes | ❌ No |
| Hoisting Behavior | ✅ Hoisted but not initialized | ✅ Hoisted and initialized with undefined |
✅ Hoisted but not initialized |
| Use Case | Variables that change | Older JavaScript code | Fixed values |
var vs let)function example() {
var a = 10;
let b = 20;
if (true) {
var a = 30; // ✅ Redeclares 'a' (var is function-scoped)
let b = 40; // ✅ Creates a new 'b' (let is block-scoped)
}
console.log(a); // Output: 30 (var leaks outside block)
console.log(b); // Output: 20 (let is block-scoped)
}
var leaks outside of the block, while let does not.
letJavaScript hoists variables declared with var, let, and const, but only var gets initialized with undefined.
let variables are hoisted but are not initialized.console.log(name); // ❌ Error! Cannot access 'name' before initialization
let name = "John";
The variable name is hoisted but remains in a "temporal dead zone" until initialized.
let✔ Use let when a variable’s value needs to change.
✔ Avoid var unless working with older code.
✔ Use const for values that should not change.
✔ Declare variables as close to their usage as possible.
@asadmukhtar