feat: reduce(func, memo, iterablish)

lib/reduce-test.ts

@@ -0,0 +1,37 @@
+import { assert } from 'chai'
+import { collect, reduce } from './'
+
+function promiseImmediate<T> (data?: T): Promise<T> {
+  return new Promise(resolve => setImmediate(() => resolve(data)))
+}
+
+function* numbers () {
+  yield 1
+  yield 2
+  yield 3
+}
+
+async function* asyncNumbers () {
+  yield 1
+  yield await promiseImmediate(2)
+  yield 3
+}
+
+const add = (num1, num2) => num1 + num2
+
+describe('reduce', () => {
+  it('reduces sync functions with sync iterators', async () => {
+    assert.equal(await reduce(add, 0, numbers()), 6)
+  })
+  it('reduces async functions with async iterators', async () => {
+    assert.equal(await reduce(add, 0, asyncNumbers()), 6)
+  })
+  it('reduces async functions with iterables', async () => {
+    assert.equal(await reduce(add, 0, [1, 2, 3]), 6)
+  })
+  it('curryable', async () => {
+    const addAble = reduce(add)
+    const addZero = addAble(0)
+    assert.equal(await addZero([1, 2, 3]), 6)
+  })
+})

lib/reduce.ts

@@ -1,3 +1,22 @@
-export function reduce () {
+import { fromIterable } from '.'
+import { Iterableish } from './types'
+export async function _reduce<T, B> (func: (B, T) => B, start: B, iterable: Iterableish<T>) {
+  let value = start
+  for await (const nextItem of fromIterable(iterable)) {
+    value = await func(value, nextItem)
+  }
+  return value
+}
 
+export function reduce<T, B> (func: (B, T) => B): (start: B) => (iterable: Iterableish<T>) => Promise<B>
+export function reduce<T, B> (func: (B, T) => B, start: B): (iterable: Iterableish<T>) => Promise<B>
+export function reduce<T, B> (func: (B, T) => B, start: B, iterable: Iterableish<T>): Promise<B>
+export function reduce (func, start?, iterable?) {
+  if (start === undefined) {
+    return (curriedStart, curriedIterable?) => reduce(func, curriedStart, curriedIterable)
+  }
+  if (iterable === undefined) {
+    return curriedIterable => reduce(func, start, curriedIterable)
+  }
+  return _reduce(func, start, iterable)
 }