4499 · Chunk

Challenge Link

Problem

Do you know lodash? Chunk is a very useful function in it, now let’s implement it. Chunk<T, N> accepts two required type parameters, the T must be a tuple, and the N must be an integer ≥ 1.

type exp1 = Chunk<[1, 2, 3], 2>    // [[1, 2], [3]]
type exp2 = Chunk<[1, 2, 3], 4>    // [[1, 2, 3]]
type exp3 = Chunk<[], 0>            // []

Solution

Approach: Accumulate a Current Chunk

Use a Current accumulator tuple that collects elements one at a time. When its length reaches N, flush it into the result and start fresh.

type Chunk<
  T extends unknown[],
  N extends number,
  Current extends unknown[] = []
> = T extends [infer Head, ...infer Tail]
  ? Current['length'] extends N
    ? [Current, ...Chunk<T, N>]
    : Chunk<Tail, N, [...Current, Head]>
  : Current extends []
    ? []
    : [Current]

Step-by-step trace for Chunk<[1, 2, 3], 2>:

Chunk<[1,2,3], 2, []>     → Current.length=0 ≠ 2, add Head=1
Chunk<[2,3], 2, [1]>      → Current.length=1 ≠ 2, add Head=2
Chunk<[3], 2, [1,2]>      → Current.length=2 = N! flush [1,2], restart with T=[3]
  ↪ [[1,2], ...Chunk<[3], 2, []>]
Chunk<[3], 2, []>         → add Head=3
Chunk<[], 2, [3]>         → T exhausted, Current=[3] non-empty → [[3]]
Result: [[1,2], [3]]  ✓

Deep Dive

The Critical “Restart with T” Detail

When flushing, the recursion restarts with the full T (which still includes Head), not Tail:

? [Current, ...Chunk<T, N>]   // ✅ pass T (includes current Head)
//          ↑
//          NOT Tail — if we passed Tail, Head would be lost

This is the single most common mistake in Chunk implementations. The flush fires before consuming the current element, so we must not advance past it.

Tuple Length as a Type-Level Counter

TypeScript tuple ['length'] yields a numeric literal type:

type T = [1, 2, 3]
type Len = T['length']  // 3 (literal, not broad `number`)

Current['length'] extends N is a zero-cost type-level comparison. This pattern — using tuple length as an integer accumulator — is the backbone of many type-level counting challenges.

Empty Tuple Detection

: Current extends []
    ? []
    : [Current]

Current extends [] matches the empty tuple literal specifically. This is preferred over Current['length'] extends 0 for readability, and both work reliably because TypeScript can narrow to empty tuple types.

Handling Edge Cases

Comparison with the JavaScript Runtime

The JavaScript equivalent:

function chunk<T>(arr: T[], n: number): T[][] {
  const result: T[][] = []
  for (let i = 0; i < arr.length; i += n) {
    result.push(arr.slice(i, i + n))
  }
  return result
}

The type-level version mirrors the imperative loop structurally: instead of i += n we use Current['length'] extends N, and instead of arr.slice we use pattern matching to gather elements into Current.

Type Parameter Defaults Hide Complexity

The Current extends unknown[] = [] default makes the accumulator invisible to callers:

// External call — clean, no accumulator exposed
type R = Chunk<[1,2,3,4,5], 2>  // [[1,2],[3,4],[5]]

Internal recursive calls pass Current explicitly to thread state through the recursion.

Key Takeaways

• “Flush then restart with T” — when a chunk fills, restart recursion with the full T (not Tail) to avoid skipping the current Head; this is the key correctness detail
• Tuple ['length'] as counter — numeric literal types on tuple lengths enable type-level equality checks without any arithmetic utility types
• extends [] for empty tuple — cleaner and more idiomatic than ['length'] extends 0 for detecting an empty accumulator
• Three-phase recursion — the “accumulate → flush → terminate” pattern is reusable for any type-level grouping or batching problem
• Default type parameters — using = [] on Current keeps the public API clean while threading mutable state through recursive calls