What is the Difference Between let, const, and var in JavaScript?

When preparing for JavaScript interviews, one of the most common questions you’ll encounter is about the differences between let, const, and var. Understanding these variable declarations is crucial for writing clean, efficient, and bug-free code.

1. Scope

• var: Function-scoped (visible within the entire function).
• let & const: Block-scoped (visible only within {} blocks like if, for, etc.).

Example:

if (true) {
  var varVar = "I'm var";
  let letVar = "I'm let";
}
console.log(varVar); // Works (function-scoped)
console.log(letVar); // Error (block-scoped)

2. Hoisting

• var: Hoisted and initialized as undefined.
• let & const: Hoisted but not initialized (Temporal Dead Zone error if accessed early).

Example:

console.log(a); // undefined (var)
console.log(b); // ReferenceError (let/const)
var a = 10;
let b = 20;

3. Reassignment & Redeclaration

• var: Can be redeclared and updated.
• let: Can be updated but not redeclared.
• const: Cannot be updated or redeclared (must be initialized).

Example:

var x = 1; var x = 2; // Allowed
let y = 1; let y = 2; // Error
const z = 1; z = 2;   // Error

4. Best Practices

• Use const by default (for immutable values).
• Use let for variables that need reassignment.
• Avoid var (legacy, prone to scope leaks).

Interview Tip

Interviewers often ask: "Why should you avoid var?"
Answer: Due to function-scoping and hoisting quirks, var can lead to unintended bugs, especially in loops and closures.