TypeScript implementation of cartessian product which infers its type perfectly.

cartessian.ts

type CartessianType<T> =
  T extends [] ? 
    [] :
  T extends [(infer R)[], ...infer Rest] ?
    [R, ...CartessianType<Rest>] :
  T extends (infer R)[][] ?
    [R] : 
  never

function cartessianProduct<T extends unknown[]>(...sets: T): CartessianType<T>[]
function cartessianProduct(s: any[], ...rest: any[]): any[][] {
  if (rest.length === 0) return s.map((e: any) => [e])

  const pre = cartessianProduct(s)
  const post = cartessianProduct(rest[0], ...rest.slice(1))
  const result: any[] = []
  
  for (const p of pre) {
    for (const q of post) {
      result.push([p[0], ...q as any[]])
    }
  }
  return result
}