What Is a Variable?

Imagine building a game that remembers a player’s name and score. You wouldn’t want to type those values into every line of code. Instead, you’d give each piece of information a name and let JavaScript remember it.

That’s what a variable does. It gives you a way to store a value and use it throughout your program.

let playerName = "Alex";
let score = 10;

console.log(playerName);
console.log(score);

Here, playerName stores text and score stores a number. You can refer to either value by its name.

Store Values That Can Change

Use let when a value may need to change as your program runs. Scores, counters, and changing messages are good examples.

let score = 10;

score = 20;

console.log(score);

The first line creates the variable. The second line assigns a new value. You don’t need to write let again when updating an existing variable.

You can also use the current value to calculate the next one.

let score = 10;

score = score + 5;

console.log(score); // 15

Keep a Value Consistent

Some values shouldn’t be reassigned after you create them. A website’s name or a fixed tax rate might be examples.

Use const when you don’t plan to assign a different value to the variable.

const websiteName = "mC0de";

console.log(websiteName);

If you try to reassign a constant, JavaScript reports an error.

const websiteName = "mC0de";

websiteName = "Another Site";
// TypeError: Assignment to constant variable.

Choose Names That Make Sense

Clear names make your code easier to understand. Compare let x = 25; with let studentAge = 25;. The second tells you what the number means.

const firstName = "Jordan";
let totalPoints = 100;
const favoriteLanguage = "JavaScript";

JavaScript Naming Rules

• Names can contain letters, numbers, underscores, and dollar signs.
• Names cannot begin with a number.
• Spaces and hyphens aren’t allowed in ordinary variable names.
• Names are case-sensitive: score and Score are different.
• Reserved words such as const cannot be variable names.

Most JavaScript developers use camelCase for variable names: start with a lowercase word, then capitalize the beginning of each following word.

Store Text, Numbers, and More

Variables can hold different kinds of information. For now, let’s explore text, numbers, and true-or-false values.

const studentName = "Taylor";
let lessonsCompleted = 2;
const isLearning = true;

console.log(studentName);
console.log(lessonsCompleted);
console.log(isLearning);

Quotation marks identify text. Numbers don’t need quotation marks, and true and false are special Boolean values.

We’ll explore these data types more closely in Lesson 03.

Use Variables to Build Messages

Once your values have names, you can combine them to create useful messages.

const name = "Alex";
let points = 15;

console.log(name + " has " + points + " points.");

The + symbol can join strings together. This is called concatenation.

Another useful option is a template literal, which uses backticks and places values inside ${...}.

const name = "Alex";
let points = 15;

console.log(`${name} has ${points} points.`);

Both examples produce the same message. Template literals are especially convenient when a sentence contains several variables.

Build Good Habits Early

Variables are simple, but choosing how to use them makes a big difference as your projects grow.

• Use const by default when reassignment isn’t needed.
• Use let when the value must change.
• Give variables descriptive names.
• Use camelCase consistently.
• Avoid creating multiple variables that mean the same thing.
• Keep related calculations easy to follow.
• Use console.log() to inspect values while learning.

You’ll use variables in almost every JavaScript project, from simple buttons to full interactive applications.