In this post, we will learn the difference between let and var, and const in JavaScript.This is a frequently asked question in JavaScript interviews for beginners. Let's dive into it.
JavaScript provides three different ways to declare variables: let, var, and const. Each of these has distinct behaviors and scoping rules.
Difference Between let vs var vs const JavaScript
Below is a table that compares let, var, and const in JavaScript, followed by an example to demonstrate their differences.Aspect | var | let | const |
---|---|---|---|
Scope | Function scope | Block scope | Block scope |
Hoisting | Initialized with undefined |
Not initialized | Not initialized |
Re-declaration | Allowed within the same scope | Not allowed within the same block | Not allowed within the same block |
Re-assignment | Allowed | Allowed | Not allowed |
Usage | General Variables | Variables with block scope requirements | Constants or references that shouldn't change |
let:
- let was introduced in ECMAScript 6 (ES6) and provides block scope.
- Variables declared with let are not hoisted, and accessing them before the declaration results in a ReferenceError.
- let allows reassignment of the variable's value, making it useful for cases where the variable needs to change its value over time. It must be initialized when declared; otherwise, it will result in a SyntaxError.
- let introduces the concept of the Temporal Dead Zone (TDZ), which prevents variables from being accessed before the declaration within their block.
var:
- var is the oldest way to declare variables in JavaScript and has function scope.
- Variables declared with var are hoisted to the top of their function scope, and they can be accessed before the declaration (resulting in undefined).
- var allows reassignment of the variable's value, which can sometimes lead to unintended bugs and issues in larger codebases.
- Unlike let and const, var does not require explicit initialization when declared.
const:
- const was introduced in ES6 and provides block scope, similar to let.
- Variables declared with const cannot be reassigned after their declaration. However, it does not make the variable itself immutable; it just prevents reassignment. It must be initialized when declared; otherwise, it will result in a SyntaxError.
- const is useful when you want to ensure that the variable remains constant throughout its lifecycle.
Example
Below is an example to illustrate the differences:
// Using var
function varExample() {
if (true) {
var x = 5; // x can be accessed outside this block
}
console.log(x); // Outputs 5
}
// Using let
function letExample() {
if (true) {
let y = 10; // y can't be accessed outside this block
}
console.log(y); // ReferenceError: y is not defined
}
// Using const
function constExample() {
const z = 20; // z must be assigned at declaration
z = 30; // TypeError: Assignment to constant variable
console.log(z);
}
let has a block scope, so y is only accessible within the block where it's defined.
const also has block scope, but it must be initialized when declared, and its value cannot be changed once assigned, as demonstrated with z.
Conclusion
In modern JavaScript development, it is generally recommended to use let and const over var. Use const when you want to ensure that the variable remains constant and use let when you expect the variable's value to change over time. Avoid using var in ES6 projects to prevent potential issues caused by hoisting and function scope. By understanding the differences between let, var, and const, developers can write more maintainable and bug-free JavaScript code.
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