The let
and const
keywords were introduced in ECMAScript 6 (ES6) to declare variables in JavaScript with block scope. Block scope means that the variable is only accessible within the block of code where it is defined. This is in contrast to the var
keyword, which has function scope or global scope.
-
let
keyword:-
Variables declared with
let
can be reassigned. - They are block-scoped, meaning their scope is limited to the block (enclosed by curly braces) in which they are defined.
- Example:
if (true) { let x = 10; console.log(x); // 10 } console.log(x); // ReferenceError: x is not defined
-
Variables declared with
-
const
keyword:-
Variables declared with
const
are also block-scoped. -
Constants declared with
const
cannot be reassigned once they are initialized. -
It's important to note that when
const
is used with objects or arrays, the reference to the object or array cannot be changed, but the properties or elements inside it can be modified. - Example:
const pi = 3.14; pi = 3.14159; // Error: Assignment to constant variable. const arr = [1, 2, 3]; arr.push(4); // This is allowed, as it modifies the array in place.
-
Variables declared with
Using let
and const
helps in writing more maintainable and less error-prone code by providing better control over variable scoping and reassignment. It also helps prevent unintended variable reassignments, making your code more predictable.