JavaScript lesson 17: Arrays and indexes
US version. US grades and US spelling. · UK version of this lesson
An array holds a list: const pets = ["cat", "dog", "owl"]. Each item has an index, starting at 0, so pets[0] is "cat". A complete 40-minute JavaScript lesson for beginners (Grades 7–8), from Unit 7: Arrays. It runs right here in your browser – nothing to install.
- Time
- 40 minutes + 20 min homework
- Grade
- Grades 7–8 (ages 12–13)
- You need
- A computer or tablet, paper and a pencil
- Graded
- Instantly, with explanations
For parents: how to use this lesson
You don't need to know JavaScript yourself. The lesson explains each step on screen, the way Mr Babbage, our Head of Computing, likes it: a worked example first, then a go for your child. Code questions run your child's program in the browser – graded straight away.
- Read what your child will learn together.
- Press Start the lesson below. Sit with your child for the teaching and the worked example (about 16 minutes): ask them to say what each line does.
- Let them do the practice on their own. When code goes wrong, the error message is a clue – let them read it out and try again before you help.
- Finish with the talk question: What could you store in an array for a game or a club?
Watch out for
- The first item is at index 1. Print pets[0].
- The last index is the length. pets[pets.length] is undefined.
Miss Lovelace's tip for home: ask your child to explain one line of their program to you in their own words. If they can explain it, they understand it – and every error message is a clue, not a failure.
What your child will learn
Learning goal: store a list of values in an array.
Success looks like
- I can store a list of values in an array.
- I can read and change items by their index.
- I can find the last item with
length - 1.
Key words
array · item · index · length · join
What to have ready
- A device with a web browser (the code runs in the browser – nothing to install)
- Paper for tracing
- A keyboard helps for typing code (a tablet or phone works too)
The big ideas (for the adult)
An array holds a list: const pets = ["cat", "dog", "owl"]. Each item has an index, starting at 0, so pets[0] is "cat".
pets.length is 3, so the last index is pets.length - 1. An index past the end gives undefined.
You can change an item: pets[1] = "fox". pets.join(", ") turns the array into text with ", " between the items.
Worked example – see it
const pets = ["whale", "fox", "shark", "koala"] console.log(pets[0])
It prints:
whale
The lesson, step by step
| Part | Time | How it is checked |
|---|---|---|
| Starter | 5 min | Graded by Classfern |
| Teaching input | 8 min | Read, watch and talk |
| See it, then predict | 8 min | Graded by Classfern |
| Put it in order, write it, fix it | 14 min | Graded by Classfern |
| Closure and exit ticket | 5 min | Graded by Classfern + self-check |
The lesson
It runs here on the page. No sign-up and no names – progress is kept only in this browser. Each visit picks a new mix of questions.
Homework
20 minutes: 7 questions – predict what code prints, write or fix a short program, and check the key ideas – then a short reflection. New questions every time, all graded instantly.
Practice tests
Curriculum
CSTA K–12 Computer Science Standards (2017), grades 6–8: 2-AP-11 – Create clearly named variables of different data types and operate on them.
The standard codes are CSTA's; the descriptions are our own words.
Our lessons and questions are our own. They are not written or endorsed by any government department, testing agency or exam board. JavaScript is a trademark or registered trademark of Oracle Corporation in the U.S. and other countries; Classfern is not affiliated with or endorsed by it.