JavaScript Fundamentals · Loops · Lesson 9 of 48
for Loops
Concept
for Loops
A for loop repeats code a set number of times using a counter. It's the most common loop when you know in advance how many times you need to repeat something.
A for loop packs three steps — where to start, when to stop, and how to move forward — into a single, easy-to-scan line, instead of spreading them across separate statements.
- Explain the core idea behind for Loops.
- Predict output before running a change.
- Test one realistic and one unusual input.
- Use the result to make the next decision.
- 1. Read one concept.
- 2. Change the example.
- 3. Run, compare, and explain.
- 4. Complete the challenge below.
Good to know
All three parts of a for loop are optional — for (;;) creates an infinite loop, easy to create by accident if you forget the increment.
Self-check before continuing
Without looking at the example, describe what changes when you modify one input in for Loops. Then reopen the editor and prove your explanation with a small test.
You are ready to continue when you can predict, test, and explain the result.
Your turn
Use a for loop to print the numbers 10 down to 1.
Ask about this lesson or your next move. The guide explains the path without taking the work away from you.
Ready for the next lesson?
Mark this lesson complete when you have finished the exercise.
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