JavaScript lesson 16: Arrow functions and scope
UK version. UK school years and UK spelling. · US version of this lesson
An arrow function is a short way to write a function: const double = (n) => n * 2. With no braces, it returns the value after =>. A complete 40-minute JavaScript lesson for beginners (Year 8, KS3), from Unit 6: Functions. 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: Why might it be useful that variables inside a function stay inside it?
Watch out for
- A let variable inside an if can be used after it. Try it: ReferenceError.
- An arrow function needs return. Without braces, it returns automatically.
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 write arrow functions.
Success looks like
- I can write arrow functions.
- I can explain block scope for
letandconst. - I can avoid two variables with the same name getting mixed up.
Key words
arrow function · scope · block · local · global
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 arrow function is a short way to write a function: const double = (n) => n * 2. With no braces, it returns the value after =>.
A variable made with let or const inside { } exists only inside those braces. This is called its scope. Outside, the name is unknown.
A variable inside a function with the same name as one outside is a DIFFERENT variable. Changing it doesn't change the outside one.
Worked example – see it
const hi = (name) => "Hi " + name
console.log(hi("Jade"))It prints:
Hi Jade
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.