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Introduction to JavaScript

JavaScript (JS) is one of the most widely used programming languages in the world. It powers interactivity on websites, controls dynamic content, and even runs on servers (via Node.js).

What Is JavaScript?

  • JavaScript is a high-level, interpreted programming language.
  • It’s most commonly used to add interactivity to web pages.
  • It runs directly in the browser (like Chrome, Firefox, or Edge) — no compilation required.
  • It can also run on servers or in standalone programs using Node.js.

You can run JavaScript:

  • In the browser console (press F12 → Console tab) (we'll be using this for the majority of our JS)
  • In online editors (like Replit or JSFiddle)
  • Using Node.js on your computer

Variables

Variables are used to store data. You can create them using let, const, or var.

let name = "Alice";
const pi = 3.14159;
var age = 25;

console.log(name); // Output: Alice
  • let: can be reassigned
  • const: cannot be reassigned
  • var: older keyword (avoid in modern JS)

Data Types

Common JavaScript data types include:

Type Example Description
String "Hello" Text data
Number 42, 3.14 Numeric values
Boolean true, false True/false logic
Array [1, 2, 3] Ordered list of values
Object { name: "Alice", age: 25 } Key-value pairs 
let student = {
  name: "Jordan",
  enrolled: true,
  grades: [85, 90, 92]
};

console.log(student.name);      // "Jordan"
console.log(student.grades[1]); // 90

Operators

JavaScript supports arithmetic, comparison, and logical operators.

let x = 10;
let y = 3;

console.log(x + y);  // 13
console.log(x - y);  // 7
console.log(x * y);  // 30
console.log(x / y);  // 3.333...
console.log(x % y);  // 1

console.log(x > y);   // true
console.log(x === y); // false
console.log(x !== y); // true

Conditional Statements

Use if, else if, and else to make decisions in your code.

let score = 85;

if (score >= 90) {
  console.log("Excellent!");
} else if (score >= 70) {
  console.log("Good job!");
} else {
  console.log("Keep trying!");
}

Loops

Loops allow you to repeat code multiple times.

// For loop
for (let i = 0; i < 5; i++) {
  console.log("Loop number:", i);
}

// While loop
let count = 0;
while (count < 3) {
  console.log("Counting:", count);
  count++;
}

Functions

Functions are reusable blocks of code that perform a specific task.

function greet(name) {
  console.log("Hello, " + name + "!");
}

greet("Alice");
greet("Bob");

You can also use arrow functions (a newer syntax):

const add = (a, b) => a + b;

console.log(add(3, 5)); // 8

Arrays

Arrays are ordered lists of values.

let colors = ["red", "green", "blue"];

console.log(colors[0]); // "red"
colors.push("purple");
console.log(colors);    // ["red", "green", "blue", "purple"]

for (let color of colors) {
  console.log(color);
}

Objects

Objects group related data and behavior together.

let car = {
  make: "Toyota",
  model: "Camry",
  year: 2020,
  honk: function() {
    console.log("Beep beep!");
  }
};

console.log(car.make); // "Toyota"
car.honk();            // "Beep beep!"

User Input (Browser Only)

In the browser, you can get input from users using prompt() and display messages with alert().

let name = prompt("What is your name?");
alert("Hello, " + name + "!");

Note: These only work in browsers, not in Node.js.

Next Steps

Once you’re comfortable with these examples, move on to the challenges.md file for your next tasks.

Recommended Resources: