filter

Keep the items that pass a test. Same items, fewer of them.

filter runs your test once per item and hands back a new array of the ones that passed: same items, same order, input untouched.

The function is a question about one item: is it active, did it pass, is it in this topic. Combine with .length to count and with map to reshape what survived.

Shape

const kept = items.filter((item) => booleanTest);

Patterns

```
results.filter((r) => r.correct)
```
Keep by a boolean field.
```
results.filter((r) => r.correct).length
```
Count matches.

results.filter((r) => !r.answered).map((r) => r.questionId)

Filter then map: narrow first, then reshape.

values.filter((v) => v !== null && v !== undefined)

Be precise about what you drop.

Pitfalls

filter wants a boolean. Returning the item itself, or a transformed value, keeps everything truthy and confuses readers.
filter(Boolean) drops 0, "" and false along with null and undefined. Fine for strings, wrong for scores.
If you only need the first match, find stops early. filter scans everything.

Exercises

Write the function, run it against the hidden cases. Cmd or Ctrl + Enter runs, Tab indents, Esc then Tab leaves the editor. 0 / 6 solved.

1. Passed results solved

Return only the results where passed is true.

Example

passedResults(
  [
    { id: 1, passed: true },
    { id: 2, passed: false },
    { id: 3, passed: true }
  ]
)

→ [{ id: 1, passed: true }, { id: 3, passed: true }]

2. Count correct solved

Return how many results have correct: true.

Example

countCorrect([{ correct: true }, { correct: false }, { correct: true }])

→ 2

3. Unanswered ids solved

Return the questionId of every result where answered is false, in input order.

Example

unansweredIds(
  [
    { questionId: "q3", answered: false },
    { questionId: "q1", answered: true },
    { questionId: "q2", answered: false }
  ]
)

→ ["q3", "q2"]

4. Active in topic solved

Return the students who are active and whose topics list includes the given topic.

Example

activeInTopic(
  [
    { name: "Ana", active: true, topics: ["vitals", "safety"] },
    { name: "Bo", active: false, topics: ["vitals"] },
    { name: "Cy", active: true, topics: ["safety"] }
  ],
  "vitals"
)

→ [{ name: "Ana", active: true, topics: ["vitals", "safety"] }]

5. Drop only empties solved

Remove null, undefined and empty strings from the list. Everything else stays, including 0 and false, which a truthiness check would wrongly drop.

Example

dropEmpties([0, "", null, "a", false, undefined, 5])

→ [0, "a", false, 5]

6. Stale documents solved

Return the documents whose updatedAt (an ISO string) is strictly before the given cutoff (also an ISO string). A document updated exactly at the cutoff is not stale. Keep input order.

Example

staleDocuments(
  [
    { id: 1, updatedAt: "2026-09-01T00:00:00Z" },
    { id: 2, updatedAt: "2026-09-20T00:00:00Z" },
    { id: 3, updatedAt: "2026-09-09T23:59:59Z" }
  ],
  "2026-09-10T00:00:00Z"
)

→ [
  { id: 1, updatedAt: "2026-09-01T00:00:00Z" },
  { id: 3, updatedAt: "2026-09-09T23:59:59Z" }
]

Next lesson reduce

Fold a list into one value: a number, an object, or a new list.