JavaScript Variables (let and const)

A variable is a named box that stores a value so you can use it later: a user’s name, a score, a list of products. Variables are the most basic building block of every program.

Declaring a variable with let#

let score = 0;
console.log(score); // 0

score = 10;
console.log(score); // 10

score = score + 5;
console.log(score); // 15

• let creates (declares) a variable.
• = assigns a value. In programming, = doesn’t mean “equals”; it means “put this value into the box”.
• After declaring a let variable, you can change its value as often as you like.

score = score + 5 reads as: take the current value of score, add 5, and store the result back in score.

Constants with const#

const siteName = "VitalStack";
const pi = 3.14159;

console.log(siteName, pi);

// siteName = "Other"; // TypeError: Assignment to constant variable.

const creates a variable that can’t be reassigned. Try uncommenting the last line: it causes an error.

Which one to use?#

**Use const by default.** Switch to let only when you know the value will change (a counter, a running total, a variable updated in a loop). This makes code easier to understand: when you see const, you know that name always refers to the same value.

const taxRate = 0.18;     // never changes
let total = 0;            // will change

total = total + 100;
total = total + 250;
console.log(`Total with tax: ${total * (1 + taxRate)}`);

const with arrays and objects#

const stops you from reassigning the variable, but the contents of an array or object can still change:

const cart = ["pen"];
cart.push("notebook");     // allowed: changing the contents
console.log(cart);         // ["pen", "notebook"]

// cart = ["eraser"];      // error: reassigning the variable

Declaring without a value#

let username;
console.log(username); // undefined

username = "rohan_k";
console.log(username);

A variable declared without a value holds undefined. A const must be given a value immediately.

Several variables at once#

let x = 1, y = 2, z = 3;
console.log(x + y + z);

This works, but one declaration per line is easier to read.

Naming rules#

Names can contain letters, digits, _ and $, but:

• Can’t start with a digit: 1st is invalid, first is fine.
• Can’t contain spaces or hyphens: user-name is invalid.
• Can’t be a reserved word like let, if, function or class.
• Are case-sensitive: total and Total are different.

Naming conventions#

JavaScript developers use camelCase: start lowercase, capitalise each following word.

let firstName = "Meera";
let totalPrice = 1299;
let isLoggedIn = true;
const MAX_UPLOAD_MB = 10; // fixed configuration values are often UPPER_CASE

Choose names that say what the value is: price beats p, userEmails beats arr. Booleans read well with is, has or can: isOpen, hasAccess.

Swapping and updating values#

let a = "first";
let b = "second";

let temp = a;
a = b;
b = temp;

console.log(a, b); // "second" "first"

let count = 5;
count += 2;  // same as count = count + 2
count -= 1;  // count = count - 1
count++;     // add 1
console.log(count); // 7

The old var keyword#

Before 2015, JavaScript only had var. You’ll see it in older code:

var oldStyle = "still works";
console.log(oldStyle);

var has confusing behaviour: it ignores block boundaries like if and for, and can be used before it’s declared without an error. let and const fixed these problems. **Don’t use var in new code.** You’ll understand the details in the Scope lesson.

Common mistakes#

• **Reassigning a const** → TypeError: Assignment to constant variable.
• **Declaring the same let twice** in the same block → SyntaxError: Identifier has already been declared. Declare once, then assign without let.
• Using a variable before declaring it → ReferenceError.
• Meaningless names like x1, data2, temp3 in real code.

Exercise#

1. Create const variables for your name and birth year, and a let for the current year. Print your age.
2. Create let balance = 1000. Subtract a purchase of 250 with -= and add a refund of 50 with +=. Print the result.
3. Swap the values of two variables using a third one.
4. Try reassigning a const and read the error message in the output.

Summary#

• Variables store values; = assigns a value.
• Use const by default, and let when the value must change.
• Name variables in camelCase with descriptive names.
• Avoid var in new code.

Next, you’ll learn the different types of data a variable can hold.