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Table of Contents

Function Scope vs. Block Scope

Hoisting Behavior

Reassignment and Mutability

When to Use Which?

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Difference between `var`, `let`, and `const` declarations in JavaScript

Robert Michael Kim

Jul 08, 2025 am 02:21 AM

variable declaration

In JavaScript, the main differences between var, let and const are scope, enhancement behavior, and reassignability. var is the function scope, while let and const are block scope; variables declared by var will be promoted and initialized to undefined, while let and const are promoted but will not be initialized, and access will be errors (temporary dead zone); var and let allow reassignment, while const does not allow reassignment, but the content of the mutable object can be modified. When using it, use const first. Use let when reassigning values ??to avoid using var.

Difference between `var`, `let`, and `const` declarations in JavaScript

In JavaScript, var , let , and const are all used to declare variables, but they behave differently in terms of scope, hoisting, and reassignment. Understanding these differences is essential for writing clean and bug-free code.

Function Scope vs. Block Scope

The main difference between var and let / const lies in their scope .

var is function-scoped , meaning a variable declared with var is accessible throughout the function it's declared in — not just the block where it was created.

Example:
```
 if (true) {
  var x = 10;
}
console.log(x); // Outputs: 10
```
let and const are block-scoped , so they're only accessible within the block {} they were declared in.
Example:
```
 if (true) {
  let y = 20;
}
console.log(y); // ReferenceError
```

This makes let and const safer to use because they prevent accidental access or modification from outside the intended scope.

Hoisting Behavior

All three declarations are hoisted, but they behave differently:

Variables declared with var are hoisted and initialized with undefined , which means you can access them before declaration (though the value will be undefined ).
Example:
```
 console.log(a); // undefined
var a = 5;
```
let and const are also hoisted, but not initialized . Accessing them before declaration results in a ReferenceError . This period is known as the Temporal Dead Zone (TDZ) .
Example:
```
 console.log(b); // ReferenceError
let b = 10;
```

So, even though all three are hoisted, using let and const encourages better coding practices by enforcing variable declaration before usage.

Reassignment and Mutability

Here's how each behaves when it comes to changing values:

var and let allow reassignment :

 let name = "Alice";
name = "Bob"; // Allowed

const does not allow reassignment , but the content of reference types like objects and arrays can still be changed:
```
 const arr = [1, 2];
arr.push(3); // Allowed
arr = []; // TypeError: Assignment to constant variable.
```

So const doesn't make values ??immutable, just prevents reassigning the variable identifier.

When to Use Which?

A few practical guidelines:

Use const by default unless you know the variable needs to be reassigned.
Use let when you need to reassign values, such as in loops or counters.
Avoid var in modern JS unless you're maintaining legacy code.

You'll rarely have a reason to use var anymore since let and const provide more predictable behavior.

That's the core of how var , let , and const different. The key takesaways are around scope, hoisting, and mutability — and knowing when to choose one over the others helps avoid bugs and confusion.

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Difference between `var`, `let`, and `const` declarations in JavaScript Jul 08, 2025 am 02:21 AM

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Difference between `var`, `let`, and `const` declarations in JavaScript

Function Scope vs. Block Scope

Hoisting Behavior

Reassignment and Mutability

When to Use Which?

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