1. What is this?
A data structure organises values so you can use them efficiently. An array is the simplest: an ordered list where each slot has an index (position) starting at 0. let scores = [8, 5, 9] holds three numbers; scores[0] is 8; scores.length is 3. A loop visits each slot in turn.
2. Why do we use it?
Scores, todos, messages — ordered collections are everywhere. Arrays plus loops process them.
3. Syntax breakdown
let scores = [8, 5, 9];
Creates an array of three numbers.
scores[0]
First item (index 0). scores[2] is the last here.
scores.length
How many items: 3.
5. Common mistakes
- Starting at index 1 — arrays start at 0, so the last index is length - 1.
- Reading scores[3] in a 3-item array — undefined, a classic off-by-one.
- Forgetting that length is a property (no parentheses).
Practice task
Given [4, 7, 2], print the average (total / length).
Hint
Sum with a loop, then console.log(total / scores.length).
Try it below — tests run automatically when you press Run.
Output
Press Run to see output.
Mini quiz
3 questions. Answer, then check.
1. What index holds the first array item?
2. What is scores.length?
3. What is a data structure?