1. What is this?
A loop is an instruction that repeats a block of code while a condition holds. A for loop has three parts: a counter start (let i = 0), a keep-going test (i < 5), and a step (i++ means add one). The block in { } runs once per value of i.
2. Why do we use it?
Rendering 100 rows, sending reminders, processing files — all loops. Writing the block once beats copying it 100 times.
3. Syntax breakdown
for (let i = 0; i < 5; i++) {Start i at 0. Keep going while i < 5. After each pass, add 1 to i.
console.log("Step " + i);Runs 5 times with i = 0,1,2,3,4.
}
Closes the repeated block.
5. Common mistakes
- Off-by-one: i < 5 runs 5 times from 0; i <= 5 runs 6 times. Check your start and test.
- Forgetting i++ creates an endless loop — the counter never moves.
- Using the counter outside the loop expecting it to keep its last meaning.
Practice task
Log the 3-times table from 3 x 1 to 3 x 5, one per line (e.g. "3 x 1 = 3").
Hint
Inside the loop use (3 * i) and build the string with +.
Try it below — tests run automatically when you press Run.
Output
Press Run to see output.
Mini quiz
3 questions. Answer, then check.
1. How many times does for (let i = 0; i < 5; i++) run?
2. What does i++ do?
3. Where does the repeated code go?