JavaScript lesson 19: Looping through arrays
US version. US grades and US spelling. · UK version of this lesson
for (const x of arr) { … } runs the block once for each ITEM, with x holding the item. 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 is easier to read: for…of or the index loop? When do you need each?
Watch out for
for…ofgives the indexes. Print x inside the loop.i <= arr.lengthvisits every item. It goes one past the end.
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: visit every item with for…of.
Success looks like
- I can visit every item with
for…of. - I can use an index loop when you need the position.
- I can total, count and filter the items of an array.
Key words
for…of · iterate · map · filter · accumulator
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)
for (const x of arr) { … } runs the block once for each ITEM, with x holding the item.
When you need the position too, use for (let i = 0; i < arr.length; i++) and read arr[i].
The loop patterns from Unit 5 work on arrays: totals, counts, the biggest, and building a new array with push. map and filter do some of these in one line.
Worked example – see it
const nums = [10, 4, 7, 2]
for (const n of nums) {
console.log(n * 2)
}It prints:
20 8 14 4
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-12 – Design and iteratively develop programs with control structures incl. nested loops and compound conditionals.
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.