Learn the basics (quick lessons)

Use this page in two ways:

• Teach: read the lesson + show examples.
• Practice: students answer questions, then click Check Answers.

1) Variables: var, let, const

let creates a block-scoped variable you can change. const creates a block-scoped variable you cannot reassign.

let score = 0;
score = score + 1; // OK

const name = "Aylin";
// name = "Sam"; // Not allowed (reassignment)

2) Data types (beginner)

• string → text like "Hello"
• number → 10, 3.14
• boolean → true / false
• undefined → declared but not assigned
• null → intentional “no value”

typeof "Hello"   // "string"
typeof 10        // "number"
typeof true      // "boolean"
let x;           // x is undefined
let y = null;    // y is null

3) Operators

=== checks value + type. (Recommended for beginners.)

2 == "2"   // true (value only)
2 === "2"  // false (value + type)

4) Arrays

An array is an ordered list. Use push() to add to the end and pop() to remove from the end.

let nums = [1, 2, 3];
nums.push(4); // [1,2,3,4]
nums.pop();   // [1,2,3]

5) Loops

for is great when you know how many times to repeat. while repeats while a condition is true.

for (let i = 0; i < 3; i++) {
  console.log(i);
}

let n = 3;
while (n > 0) {
  console.log(n);
  n--;
}

6) Functions

A function is a reusable block of code. return stops the function and sends a value back.

function add(a, b) {
  return a + b;
}
let result = add(2, 3); // 5

7) DOM basics (browser)

const title = document.getElementById("title");
title.textContent = "Hello!";