JavaScript lesson 3: Variables: let and const

UK version. UK school years and UK spelling. · US version of this lesson

A variable is a box with a label. let score = 10 makes a box called score and puts 10 in it. A complete 40-minute JavaScript lesson for beginners (Year 8, KS3), from Unit 2: Variables and numbers. 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 is it called a variable? When would you choose const?

Watch out for

A variable keeps its first value. Trace it with a table: write each new value in turn.
console.log("score") shows the value. Quote marks print the word; no quote marks print the value.

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 value in a variable with let or const.

Success looks like

I can store a value in a variable with let or const.
I can change a variable and predict its new value.
I can choose between let and const.

Key words

variable · let · const · value · assignment · trace table

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)

A variable is a box with a label. let score = 10 makes a box called score and puts 10 in it.

A variable holds only its newest value: after score = 15, the 10 is gone. score = score + 1 (or score += 1, or score++) works out the right side first, then stores it.

Use const for a value that never changes. Changing a const stops the program with a TypeError. Good names describe the value, like highScore – no spaces, and not starting with a digit.

Worked example – see it

const name = "Max"
console.log("name")
console.log(name)

It prints:

name
Max

The lesson, step by step

40 minutes in 5 parts
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

All JavaScript practice tests · All 24 JavaScript lessons

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; and to design, use and evaluate computational abstractions that model the state and behaviour of real-world problems and physical systems.

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.