@johngw/stream/test/transformers/filter.test.ts

Reactive programming tools using the WHATWG Streams API.

import { fromTimeline } from '@johngw/stream-jest'
import { fromCollection } from '@johngw/stream/sources/fromCollection'
import { filter } from '@johngw/stream/transformers/filter'
import { write } from '@johngw/stream/sinks/write'

test('filters unwanted values', async () => {
  await expect(
    fromTimeline<number>(`
    -0-1-2-3-4-5-6-7-8-9-|
    `).pipeThrough(filter((x) => x % 2 === 0))
  ).toMatchTimeline(`
    -0---2---4---6---8----
  `)
})

test('using type guards', async () => {
  type A = { type: 'a' }
  type B = { type: 'b' }
  type AB = A | B

  await fromCollection<AB>([{ type: 'a' }, { type: 'b' }])
    .pipeThrough(filter((chunk): chunk is B => chunk.type === 'b'))
    .pipeTo(
      write((chunk) => {
        // @ts-expect-error This comparison appears to be unintentional because the types '"b"' and '"a"' have no overlap.
        chunk.type === 'a'
        chunk.type === 'b'
      })
    )
})