JavaScript lesson 18: Changing arrays: push, pop and more
US version. US grades and US spelling. · UK version of this lesson
push(x) adds x to the END and gives back the new length. pop() removes the last item and gives it back. unshift and shift do the same at the START. A complete 40-minute JavaScript lesson for beginners (Grades 7–8), from Unit 7: Arrays. 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 array method surprised you most today?
Watch out for
pushgives back the array. console.log(list.push(x)) prints the new length.sort()sorts numbers by size. Try [10, 9, 1].sort().
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: add and remove items with push, pop, shift and unshift.
Success looks like
- I can add and remove items with
push,pop,shiftandunshift. - I can check for an item with
includesandindexOf. - I can sort numbers correctly.
Key words
push · pop · includes · indexOf · sort · method
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)
push(x) adds x to the END and gives back the new length. pop() removes the last item and gives it back. unshift and shift do the same at the START.
A const array can still change: const only means the name always points to the same array.
includes(x) is true or false; indexOf(x) gives the position or -1. Careful: sort() on its own sorts as TEXT, so 10 comes before 9. For numbers use sort((a, b) => a - b).
Worked example – see it
const list = ["mango", "grape", "bread"]
console.log(list.includes("grape"), list.includes("cherry"))It prints:
true false
The lesson, step by step
| 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
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; 2-AP-16 – Use existing code, media and libraries in original programs, with attribution.
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.