JavaScript lesson 14: Your own functions

US version. US grades and US spelling. · UK version of this lesson

function greet() { … } saves some code under a name. Nothing runs yet. greet() CALLS it: the code inside runs, then the program carries on after the call. A complete 40-minute JavaScript lesson for beginners (Grades 7–8), from Unit 6: Functions. 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: Which part of an earlier program could have been a function?

Watch out for

Writing a function runs it. Comment out the call: nothing happens.
greet without brackets calls it. Only greet() runs it.

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: write a function and call it.

Success looks like

I can write a function and call it.
I can explain that defining a function doesn't run it.
I can use a function to avoid repeating code.

Key words

function · call · define · parameter · reuse

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)

function greet() { … } saves some code under a name. Nothing runs yet. greet() CALLS it: the code inside runs, then the program carries on after the call.

You can call a function as many times as you like – that is the point: write it once, use it often.

A function can take a value in brackets: function stars(n) makes a parameter n, and stars(3) runs it with n = 3.

Worked example – see it

function welcome(name) {
  console.log("Welcome, " + name)
}
welcome("Ivy")
welcome("Leo")

It prints:

Welcome, Ivy
Welcome, Leo

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-14 – Create procedures with parameters to organize and reuse code; 2-AP-13 – Decompose problems to help design, implement and review programs.

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.