JavaScript lesson 3: Variables: let and const

US version. US grades and US spelling. · UK 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 (Grades 7–8), from Unit 2: Variables and numbers. 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: 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

Learning goal: 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	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

All JavaScript practice tests · All 24 JavaScript lessons

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.