Matt Lawrence ☕

Posted on Jul 15 • Originally published at htmlallthethings.com

What are var, let, and const in JavaScript?

#webdev #javascript #programming #tutorial

In JavaScript, var, let, and constare used to declare variables—but they differ in how they behave when it comes to scope, hoisting, and mutability. Choosing the right one helps prevent bugs and makes your code easier to reason about.

In modern development, let and const are preferred. var is mostly reserved for legacy code or situations requiring function-scoped variables. Many developers search “what is let var const” — this guide breaks it all down so you can write cleaner, safer JavaScript.

🔄 What's the difference between var, let, and const?

Feature	`var`	`let`	`const`
Scope	Function scope	Block scope	Block scope
Hoisting	✅ Yes (initialized as `undefined`)	✅ Yes (not initialized)	✅ Yes (not initialized)
Reassignable	✅ Yes	✅ Yes	❌ No
Redeclarable	✅ Yes	❌ No	❌ No

Understanding scope in JavaScript

Variable scope determines where a variable is accessible.

var is function-scoped — accessible throughout the entire function.
let and const are block-scoped — accessible only within the { } block they are defined in.

function example() {
    if (true) {    
        var x = 1;    
        let y = 2;  
    }  
    console.log(x); // ✅ Outputs 1  
    console.log(y); // ❌ ReferenceError
}

What do hoisting, reassignable, and redeclarable mean?

Hoisting

JavaScript moves variable declarations to the top of their scope.

With var the variable is hoisted and initialized as undefined.
With let and const the declaration is hoisted but not initialized, so using them before their declaration results in a ReferenceError.

Reassignable

Can you assign a new value to the variable after declaring it?

let and var allow reassignment.
const does not.

Redeclarable

Can you declare the same variable name again in the same scope?

var allows it.
let and const do not — which helps prevent accidental bugs.

How to choose between var, let, and const

✅ Use const by default.
Prevents accidental reassignment and communicates intent clearly.
🔁 Use let when the variable will change.
Useful in loops, state changes, toggles, and accumulations.
🚫 Avoid var unless maintaining legacy code
var still works, but its function scope and redeclaration behavior make it error-prone in modern codebases.

When to use var, let and const

✅ Use const when:

The variable should never be reassigned.
You want stable, predictable references.
You're working with arrays or objects that can be mutated but shouldn’t be reassigned entirely.

const config = { debug: false };
config.debug = true; // ✅ 
allowedconfig = {}; // ❌ Error: Assignment to constant variable

🔁 Use let when:

You need to reassign the variable later.
You're using a variable inside a block (for, if, etc.).
You want to avoid accidental redeclaration.

let score = 0;
score += 10;

🕰 Use var when:

You're working in legacy ES5 codebases.
You need function-level scope instead of block scope.
You’re supporting very old browsers.

function legacyCode() {
    if (true) {
        var total = 42;  
    }  
    console.log(total); // ✅ 42 (still accessible)
}

FAQ

Is it better to use let or const?

Use const whenever possible. It helps prevent bugs by signaling that the variable won’t change. Use let when reassignment is necessary.

Why is var discouraged in modern JavaScript?

var is function-scoped and allows redeclaration, which can lead to confusing bugs. let and const offer safer scoping and behavior.

Is let faster than var?

Not really. Performance is nearly identical in most cases. The real benefit is developer clarity and fewer bugs, not execution speed.

Can you reassign a const variable?

No — but you can change the contents of arrays or objects declared with const.

const colors = ['red', 'blue'];
colors.push('green'); // ✅ Works
colors = ['yellow'];  // ❌ Error

What happens if you use var inside a function?

It’s hoisted and scoped to the entire function, initialized as undefined no matter where you declare it.

Does let have block scope?

Yes. let only exists within the {} block where it's declared.

Can you redeclare variables with let or const?

No — attempting to redeclare in the same scope throws a SyntaxError.

let a = 1;
let a = 2; // ❌ Error

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