JavaScript lesson 17: Arrays and indexes
UK version. UK school years and UK spelling. · US 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 (Year 8, KS3), from Unit 7: Arrays. It runs right here in your browser – nothing to install.
- Time
- 40 minutes + 20 min homework
- Year
- Year 8, KS3 (ages 12–13)
- You need
- A computer or tablet, paper and a pencil
- Marked
- 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 – marked 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
We are learning to 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 | Marked by Classfern |
| Teaching input | 8 min | Read, watch and talk |
| See it, then predict | 8 min | Marked by Classfern |
| Put it in order, write it, fix it | 14 min | Marked by Classfern |
| Plenary and exit ticket | 5 min | Marked 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 marked instantly.
Practice tests
Curriculum
National curriculum in England, computing, key stage 3: pupils should be taught to use 2 or more programming languages, at least one of which is textual, to solve a variety of computational problems; make appropriate use of data structures [for example, lists, tables or arrays]; design and develop modular programs that use procedures or functions.
Contains public sector information licensed under the Open Government Licence v3.0.
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.