Generate recursive typed functions

recursive-factory.ts

import { deepEqual } from 'assert'

type RecursiveFn<T> = (xs: T[], out?: T[]) => T[]
type OperatorFn<T> = (T: T, xs: T[], out: T[]) => T | void
type FactoryFn = <T>(fn: OperatorFn<T>) => RecursiveFn<T>

const recursiveFactory: FactoryFn = <T>(fn: OperatorFn<T>) => {
  const recursive: RecursiveFn<T> = (xs, out = []) => {
    const x = xs.shift()

    const res = fn(x, xs, out)

    if (res) {
      out.push(res)
    }

    if (xs.length) {
      return recursive(xs, out)
    }

    return out
  }

  return recursive
}

interface Status {
  id: string;
  effectiveDate: string;
  type: string;
}

const reduceStatuses: OperatorFn<Status> = (x, _xs, out) => {
  if (out.some(n => n.effectiveDate === x.effectiveDate && n.type === x.type)) {
    return
  }

  return x
}

const reducer = recursiveFactory<Status>(reduceStatuses)

const input = [
  {
    id: '2e7c0c2b-9238-46cc-80ed-1e6cda05a1ca',
    effectiveDate: '2019-10-01',
    type: 'ACTIVE'
  },
  {
    id: '60a93504-23bc-4f17-a4e3-d8e069c4600c',
    effectiveDate: '2019-11-01',
    type: 'TERMINATED'
  },
  {
    id: 'b544ddde-3593-46a9-8280-8d4139bc261e',
    effectiveDate: '2019-10-01',
    type: 'ACTIVE'
  }
]

const expected = [
  {
    id: '2e7c0c2b-9238-46cc-80ed-1e6cda05a1ca',
    effectiveDate: '2019-10-01',
    type: 'ACTIVE'
  },
  {
    id: '60a93504-23bc-4f17-a4e3-d8e069c4600c',
    effectiveDate: '2019-11-01',
    type: 'TERMINATED'
  },
]

const output = reducer(input)

deepEqual(output, expected)