sort

Order a copy of the list with a comparator you control.

sort takes a comparator (a, b) that returns a negative number when a should come first, a positive number when b should, and zero when they tie. a - b sorts numbers ascending, b - a descending. Strings use a.localeCompare(b).

Two things bite everyone. First, sort with no comparator converts everything to strings, so [10, 9, 1] sorts to [1, 10, 9]. Always pass a comparator for numbers. Second, sort mutates the array in place. Copy first with [...items].sort(...), or use toSorted, which returns a new array.

Multi-key ordering is one comparator with a fallback: byPercentDesc || byNameAsc. The || moves to the next key only when the first one returns zero, a tie.

Shape

const ordered = [...items].sort((a, b) => a.score - b.score);
const ordered = items.toSorted((a, b) => b.score - a.score);

Patterns

```
[...scores].sort((a, b) => b - a)
```
Numbers descending, copy first.

[...docs].sort((a, b) => a.title.localeCompare(b.title))

Strings, properly.

[...docs].sort((a, b) => new Date(b.updatedAt) - new Date(a.updatedAt))

Dates: subtract them. Newest first.

[...students].sort((a, b) => b.percent - a.percent || a.name.localeCompare(b.name))

Two keys.

```
[...attempts].sort(byScore).slice(0, 3)
```
Top N is sort, then slice.

Pitfalls

No comparator means string order. [10, 9, 1].sort() is [1, 10, 9].
sort mutates. If the caller keeps using the original array, they now have a reordered one they did not ask for.
A comparator must return a number. Returning a boolean like a > b is wrong, because false becomes 0 and means "tie".

Exercises

Write the function, run it against the hidden cases. Cmd or Ctrl + Enter runs, Tab indents, Esc then Tab leaves the editor. 0 / 5 solved.

1. Scores descending solved

Return the scores from highest to lowest. Do not mutate the input.

Example

scoresDescending([70, 100, 85, 9])

→ [100, 85, 70, 9]

2. By title solved

Return the documents alphabetically by title, ignoring case. Do not mutate the input.

Example

byTitle(
  [
    { title: "Onboarding" },
    { title: "API guide" },
    { title: "Q3 notes" }
  ]
)

→ [
  { title: "API guide" },
  { title: "Onboarding" },
  { title: "Q3 notes" }
]

3. Newest first solved

Return the documents ordered by updatedAt (ISO strings), newest first. Do not mutate the input.

Example

newestFirst(
  [
    { id: 1, updatedAt: "2026-09-01T10:00:00Z" },
    { id: 2, updatedAt: "2026-09-20T10:00:00Z" },
    { id: 3, updatedAt: "2026-09-10T10:00:00Z" }
  ]
)

→ [
  { id: 2, updatedAt: "2026-09-20T10:00:00Z" },
  { id: 3, updatedAt: "2026-09-10T10:00:00Z" },
  { id: 1, updatedAt: "2026-09-01T10:00:00Z" }
]

4. Rank students solved

Return the students ordered by percent descending, then by name ascending for ties. Do not mutate the input.

Example

rankStudents(
  [
    { name: "Cy", percent: 90 },
    { name: "Ana", percent: 100 },
    { name: "Bo", percent: 90 }
  ]
)

→ [
  { name: "Ana", percent: 100 },
  { name: "Bo", percent: 90 },
  { name: "Cy", percent: 90 }
]

5. Top N attempts solved

Return the n attempts with the highest percent, highest first. When two attempts tie on percent, the earlier submittedAt (an ISO string) ranks higher. If there are fewer than n attempts, return them all. Do not mutate the input.

Example

topAttempts(
  [
    { id: "c", percent: 75, submittedAt: "2026-09-01T10:00:00Z" },
    { id: "a", percent: 75, submittedAt: "2026-09-02T10:00:00Z" },
    { id: "b", percent: 100, submittedAt: "2026-09-03T10:00:00Z" },
    { id: "d", percent: 50, submittedAt: "2026-09-04T10:00:00Z" }
  ],
  2
)

→ [
  { id: "b", percent: 100, submittedAt: "2026-09-03T10:00:00Z" },
  { id: "c", percent: 75, submittedAt: "2026-09-01T10:00:00Z" }
]