Variables

6. Variables#

Variables allow you to store information and use it throughout your program. In this section, you’ll learn how to define and use variables in JavaScript, understand common data types, and explore key concepts like naming conventions, typeof, and the difference between let and const.

6.1. Defining Variables#

In JavaScript, you define variables using the let keyword. The syntax looks like this:

let variableName = value;

  • let is the keyword that tells JavaScript you’re creating a variable.

  • variableName is the name you give the variable.

  • = is the assignment operator

  • value is the data you want to store in the variable.

Example:

let age = 16; // A variable named 'age' storing the number 16
let name = "Stevie"; // A variable named 'name' storing the string "Stevie"

You can also define a variable without giving it an initial value:

let favoriteColor;

This creates a variable called favoriteColor with no value assigned to it yet. We’ll come back to this idea later!

6.2. Using Variables#

Once a variable is defined, you can use it in your code:

let greeting = "Hello";
console.log(greeting); // Prints: Hello

You can also update the value of a variable:

let score = 10;
score = 20; // Updating the value of 'score' to 20
console.log(score); // Prints: 20

6.3. Common Data Types#

Variables in JavaScript can store different types of data. Here are the three most common:

String

Text enclosed in quotes (single or double)

let message = "Welcome!";

Number

Numeric values (both integers and decimals)

let price = 19.99;

Boolean

Represents :js`true` or false values, which are lowercase.

let isLoggedIn = true;

6.4. Checking Data Types#

You can use the typeof operator to check the type of a variable:

let example = "Hello";
console.log(typeof example); // Prints: string

let number = 42;
console.log(typeof number); // Prints: number

You can also use typeof like a function e.g

let example = "Hello";
console.log(typeof(example)); // Prints: string

6.5. Variable Naming#

When naming variables, you must follow these rules:

  • Variable names can only include letters, numbers, underscores (_), or dollar signs ($).

  • Variable names must start with a letter, underscore, or dollar sign.

  • Variable names cannot be reserved keywords (e.g., let, const, if).

Examples of valid variable names:

let userName;
let $amount;
let _isAvailable;

Invalid variable names:

let 123name; // Cannot start with a number
let let;     // 'let' is a reserved keyword

6.6. CamelCase Convention#

JavaScript developers often use the camelCase naming convention for variable names:

  • The first word starts with a lowercase letter.

  • Each subsequent word starts with an uppercase letter.

Examples:

let firstName;
let totalScore;
let isUserLoggedIn;

6.7. let vs const#

Aside from let, you can also define variables with const, which is short for “constant”. Variables defined with const are used when the variable’s value should not change.

Example:

const PI = 3.14;
PI = 3.15; // This will cause an error

6.8. undefined and null#

JavaScript has two special values to represent variables with no meaningful data:

undefined

A variable is undefined when it is declared but not assigned a value.

let x;
console.log(x); // Prints: undefined

null

null is explicitly assigned to represent “no value.”

let y = null;
console.log(y); // Prints: null