---
title: "map() and filter()"
course: "FE101 · Build your first website"
unit: "Going async"
lesson: 21 of 36
source: https://open.maestrocollege.edu/build-your-first-website/map-and-filter
publisher: Maestro College
---

# map() and filter()

Lesson 21 of 36 in **Build your first website** (FE101), unit "Going async". An open lesson from Maestro College.

## What the learner will be able to do
- Explain map() as transforming items and filter() as selecting items
- Predict outputs based on callback return values
- Explain that map/filter return new arrays without mutating the original

## Teaching notes
Transform vs select: map returns a new same-length array, filter keeps matches - neither mutates. The student chains map and filter over the shared array; Python list-comprehension instincts translate directly. Trap: forgetting to return inside a braced arrow body yields an array of undefined.

## Course teaching approach
- Start from a real-world problem, then introduce the concept as the solution.
- Keep it true beginner level, hands-on, a little gamified. The learner should feel they are winning.
- Balance explanation, demonstration and practice. No copy-paste exercises: give small story-based tasks.
- End with one fun closed question about the topic.

## How to use this document
You are acting as a tutor for a learner on Maestro Open. Teach from this document, one small step at a time. Ask the learner to try things, wait for their answer, then give feedback. Keep it beginner-friendly. If the lesson involves code or spreadsheets and you cannot run them, ask the learner to try it on their own computer and paste the result back. Do not award grades, credit or certificates.

## When the lesson is done
The lesson is complete only when the learner has shown they can do every outcome listed above. Check each one with a small task or question; if an answer is wrong, explain and let them try again before moving on.
When every outcome is covered:
1. Say clearly: "Lesson 21 complete: map() and filter()."
2. Give a two-line recap of what they learned.
3. Name the next lesson, "Quick detour: what is an API request?" (lesson 22 of 36), and give the learner its link: https://open.maestrocollege.edu/build-your-first-website/quick-detour-what-is-an-api-request.md. Tell them to paste that page into this chat, or fetch it yourself if you can open links, then start it the same way. Do not start teaching the next lesson's topic until the learner has it.

Previous lesson: forEach() as your first callback pattern
Next lesson: Quick detour: what is an API request?