Reverse

Problem Link

Problem

Implement the type version of Array.reverse.

type a = Reverse<['a', 'b']>       // ['b', 'a']
type b = Reverse<['a', 'b', 'c']>  // ['c', 'b', 'a']

Solution

Approach 1: Recursive Head/Tail — Prepend Last to Front

The cleanest approach: peel off the last element of the tuple and move it to the front.

type Reverse<T extends unknown[]> =
  T extends [...infer Rest, infer Last]
    ? [Last, ...Reverse<Rest>]
    : T

How it works:

• Destructure T into everything except the last element (Rest) and the last element itself (Last).
• Prepend Last to the recursively-reversed Rest.
• Base case: when T is [] or [X], it matches T (single element or empty, already reversed).

Step-by-step for Reverse<['a', 'b', 'c']>:

Step 1: Rest = ['a', 'b'], Last = 'c'  → ['c', ...Reverse<['a', 'b']>]
Step 2: Rest = ['a'],      Last = 'b'  → ['c', 'b', ...Reverse<['a']>]
Step 3: Rest = [],         Last = 'a'  → ['c', 'b', 'a', ...Reverse<[]>]
Step 4: T = [] (base case)             → ['c', 'b', 'a']

Approach 2: Accumulator Pattern (Tail-Recursive Style)

Using an accumulator avoids deep recursion by building the result incrementally:

type Reverse<T extends unknown[], Acc extends unknown[] = []> =
  T extends [infer Head, ...infer Rest]
    ? Reverse<Rest, [Head, ...Acc]>
    : Acc

How it works:

• Peel the first element (Head) off T and prepend it to the accumulator Acc.
• Since we prepend to Acc, the first element processed ends up deepest (i.e., last in the result).
• When T is empty, Acc holds the reversed tuple.

Step-by-step for Reverse<['a', 'b', 'c']>:

Call 1: T = ['a','b','c'], Acc = []         → Reverse<['b','c'], ['a']>
Call 2: T = ['b','c'],     Acc = ['a']      → Reverse<['c'], ['b','a']>
Call 3: T = ['c'],         Acc = ['b','a']  → Reverse<[], ['c','b','a']>
Call 4: T = [],            Acc = ['c','b','a'] → ['c','b','a']

This mirrors the classic functional “fold-left” pattern.

Approach 3: Spread Trick (Non-Recursive, TypeScript 4.0+)

TypeScript 4.0 added variadic tuple types. You can express reversal in terms of spread patterns, though TypeScript does NOT natively support reversing spreads in one step — you still need recursion. However, for known-length small tuples, you could hardcode overloads:

// Works only for fixed arities — impractical for the general case
type Reverse2<T extends unknown[]> =
  T extends [infer A, infer B]         ? [B, A] :
  T extends [infer A, infer B, infer C] ? [C, B, A] :
  // ... grows quickly, not scalable
  T

Approach 1 or 2 is always preferred.

Comparison

Key Takeaways

• Both [infer Head, ...infer Rest] (front destructuring) and [...infer Rest, infer Last] (back destructuring) are valid in TypeScript 4.0+.
• The accumulator pattern is useful for complex tuple transformations where you want to pass state forward.
• Reversing a tuple is a foundational operation — it’s used in challenges like FlipArguments, Palindrome, and LastIndexOf.
• TypeScript will hit recursion depth limits (~1000 levels) for very long tuples; both approaches share this limitation.