Three Ways to Declare

In JavaScript, there are three keywords used to declare variables: var, let, and const. Understanding the difference between them is a fundamental skill for any modern developer.

1. var (The Old Way)

Before 2015, var was the only way to declare variables.

var language = "JavaScript";
var language = "Python"; // var allows re-declaration without errors!

Verdict: You should generally avoid using var in modern JavaScript because it has confusing rules regarding "scope" (where the variable is available in your code) and allows accidental re-declarations which can cause terrible bugs.

2. let (The Modern Standard)

Introduced in ES6 (2015), let is the modern replacement for var. It allows you to declare a variable whose value can be reassigned later, but it prevents you from accidentally declaring the same variable twice.

let score = 10;
score = 15; // Perfectly fine! Reassigning the value.
// let score = 20; // Error! Cannot re-declare 'score'

3. const (For Constants)

Also introduced in ES6, const stands for "constant". You use it to declare variables whose values should never change.

const PI = 3.14159;
// PI = 3.14; // Error! Cannot reassign a constant.

Note: When using const with Objects and Arrays, you cannot reassign the variable itself, but you can modify the contents inside it!

Best Practice Rule of Thumb:

Always use const by default.
If you know the variable's value will need to change later (like a counter in a loop), use let.
Pretend var doesn't exist.

💡 Himanshu's Tip:

When I write code, I use const for exactly 95% of my variables. It prevents accidental bugs where you overwrite data. Only switch it to let when you hit an error telling you that you can't reassign it!

Interview Questions

What are the main differences between var and let?
Why is const the recommended default for declaring variables?
Can you modify the contents of an array declared with const?