JavaScript lesson 11: for loops
US version. US grades and US spelling. · UK version of this lesson
for (let i = 0; i < 5; i++) { … } has three parts: where i starts, the condition checked BEFORE every pass, and the step taken after each pass. A complete 40-minute JavaScript lesson for beginners (Grades 7–8), from Unit 5: Loops. 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: What would you use a loop for in a game?
Watch out for
i < 5includes 5. Count the passes in a trace table.- The loop always starts at 1. It starts where the first part says.
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: repeat code with a for loop.
Success looks like
- I can repeat code with a
forloop. - I can predict how many times a loop runs.
- I can use a loop to count, step and add up.
Key words
loop · for · iteration · counter · 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)
for (let i = 0; i < 5; i++) { … } has three parts: where i starts, the condition checked BEFORE every pass, and the step taken after each pass.
That loop runs 5 times, with i = 0, 1, 2, 3, 4. Change i < 5 to i <= 5 and it runs 6 times. You can count down with i-- or step with i += 2.
A trace table shows the loop variable and the other variables on each pass. It is the best way to check a loop by hand.
Worked example – see it
let count = 0
for (let i = 0; i < 6; i++) {
count++
}
console.log(count)It prints:
6
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.