JavaScript Variables and Data Types

Understanding Variables and Data Types

Variables are containers that store data values. In JavaScript, you can create variables using var, let, or const.

Declaring Variables

// Modern way (recommended)
let userName = "John";
const age = 25;

// Older way (avoid in new code)
var city = "New York";

Variable Naming Rules

• Must start with a letter, underscore (_), or dollar sign ($)
• Can contain letters, numbers, underscores, and dollar signs
• Cannot use reserved keywords (like let, function, etc.)
• Case-sensitive (myVar and myvar are different)

JavaScript Data Types

1. Numbers

let integer = 42;
let decimal = 3.14;
let negative = -10;

2. Strings

let singleQuotes = 'Hello';
let doubleQuotes = "World";
let templateLiteral = `Hello, ${userName}!`;

3. Booleans

let isActive = true;
let isComplete = false;

4. Arrays

let fruits = ["apple", "banana", "orange"];
let numbers = [1, 2, 3, 4, 5];

5. Objects

let person = {
    name: "Alice",
    age: 30,
    city: "Boston"
};

6. Undefined and Null

let undefinedVar;  // undefined
let nullVar = null;  // null

Checking Data Types

console.log(typeof 42);        // "number"
console.log(typeof "Hello");   // "string"
console.log(typeof true);      // "boolean"
console.log(typeof [1,2,3]);   // "object"

Practice Exercise

// Create variables for a student profile
let studentName = "Your Name";
let studentAge = 20;
let isEnrolled = true;
let subjects = ["Math", "Science", "English"];
let grades = {
    math: 85,
    science: 92,
    english: 78
};

// Display the information
console.log("Student:", studentName);
console.log("Age:", studentAge);
console.log("Enrolled:", isEnrolled);
console.log("Subjects:", subjects);
console.log("Grades:", grades);