JavaScript lesson 9: Making decisions: if and else

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

if (condition) { … } runs the block only when the condition is true. else { … } runs only when it is false – so exactly one of them runs. A complete 40-minute JavaScript lesson for beginners (Year 8, KS3), from Unit 4: Decisions. 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: Think of a decision you make every morning. Write it as if and else.

Watch out for

Both the if block and the else block run. Trace it: only one branch runs.
if (x = 5) checks whether x is 5. One = stores 5. Use ===.

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 make a decision with if and else.

Success looks like

I can make a decision with if and else.
I can write conditions with comparisons.
I can test both branches, including the boundary.

Key words

if · else · condition · branch · block · boundary

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)

if (condition) { … } runs the block only when the condition is true. else { … } runs only when it is false – so exactly one of them runs.

The braces { } show which lines belong to the if. Indenting the lines inside makes the program easier to read.

One = stores a value; === compares. Always test the boundary: if the pass mark is 50, test 49, 50 and 51.

Worked example – see it

const score = 62
if (score > 50) {
  console.log("Pass")
}
console.log("Done")

It prints:

Pass
Done

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.

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.