Implementing a Zip Function in TypeScript with Advanced Type Inference

We start with a simple JavaScript‑style zip function that merges two arrays element‑by‑element, returning an array of pairs.

function zip(target, source) {
  if (!target.length || !source.length) return [];
  const [one, ...rest1] = target;
  const [other, ...rest2] = source;
  return [[one, other], ...zip(rest1, rest2)];
}

Next we give the function a basic TypeScript signature using unknown[] for both parameters and the return type, and show two ways to declare the type – a function declaration and an arrow‑function variable.

function zip(target: unknown[], source: unknown[]): unknown[];
const zip = (target: unknown[], source: unknown[]): unknown[];

The third layer introduces advanced type programming: a recursive conditional type Zip<One, Other> extracts the first element of each tuple, builds a pair, and recurses on the rest, producing an exact tuple type for the result.

type Zip
=
  One extends [infer OneFirst, ...infer Rest1]
    ? Other extends [infer OtherFirst, ...infer Rest2]
      ? [[OneFirst, OtherFirst], ...Zip
]
      : []
    : [];

Because literal arrays are inferred as readonly tuples, we define a Mutable<T> mapped type to strip the readonly modifier.

type Mutable
= { -readonly [Key in keyof Obj]: Obj[Key] };

We then overload the zip function: a generic overload that returns Zip<Mutable , Mutable > , followed by the non‑generic implementation.

function zip
(target: Target, source: Source): Zip
, Mutable
>;

function zip(target: unknown[], source: unknown[]): unknown[] {
  if (!target.length || !source.length) return [];
  const [one, ...rest1] = target;
  const [other, ...rest2] = source;
  return [[one, other], ...zip(rest1, rest2)];
}

Finally we demonstrate usage with literal arrays (using as const ) and with regular arrays, showing that the inferred return type matches the exact paired tuple when possible.

const result = zip([1,2,3] as const, [4,5,6] as const);
// result: [[1,4],[2,5],[3,6]]

const arr1 = [1,2,3];
const arr2 = [4,'5',6];
const result2 = zip(arr1, arr2);
// result2: (number | string)[][]