JavaScript Variables

A variable is a name that refers to a value. Once you put a value in a variable, you can use the name anywhere in your code.

let greeting = "Hello, world!";
console.log(greeting);

We declared a variable called greeting, assigned it the string "Hello, world!", and then used it with console.log. Three steps, one line.

Three Ways to Declare

JavaScript has three keywords for declaring a variable: let, const, and var. They look almost identical — but the behavior is different in important ways.

let count = 0;            // a value that will change
const PI = 3.14159;       // a value that will never change
var legacy = "old style"; // the original (1995) way

count = count + 1;
console.log(count, PI, legacy);

const vs let

const declares a value that cannot be reassigned. let declares one that can. That’s the only difference.

const PI = 3.14159;
// PI = 3;  // ❌ TypeError: Assignment to constant variable.

let count = 0;
count = 1;  // ✅ fine — let allows reassignment
console.log(count);

A subtle point about const objects

const makes the binding constant, not the value. If your const holds an object or array, you can still change what’s inside it.

const user = { name: "Ada" };
user.name = "Lin";        // ✅ mutating a property is fine
console.log(user.name);   // "Lin"

// user = { name: "Other" }; // ❌ but reassigning the binding isn't

We’ll come back to this when we cover objects. For now: const says “this name will always point at the same thing”.

What About var?

var was the only way to declare a variable until 2015 (ES6). It has some surprising behaviors that caused real bugs for two decades. Then let and const were added to fix them.

You will see var in older code, tutorials, and tweets. But you shouldn’t use it in new code.

Block Scope

let and const are block-scoped: they exist only inside the { } they were declared in.

{
  let message = "inside the block";
  console.log(message);  // "inside the block"
}

// console.log(message);  // ❌ ReferenceError — message doesn't exist out here

This is a feature: variables can’t leak out and pollute the rest of your program. var doesn’t do this — it’s function-scoped, which is why it caused so many bugs.

Naming Variables

The naming rules are the same ones we covered in JavaScript Syntax:

• Start with a letter, _, or $.
• Then letters, digits, _, or $.
• Can’t be a reserved keyword.
• Case-sensitive.

And one convention: use camelCase.

let userName = "Ada";
let totalPrice = 29.99;
let isLoggedIn = true;

Try It Yourself

// Declare `greeting` here and log it.

const greeting = "hello";
console.log(greeting);

Test Your Knowledge

Up Next

Now you know how to store data. Next, two short lessons that zoom in on let and const specifically.