merge ensuring that all generators are fully consumed

exhaust.mjs

import { setTimeout } from 'timers/promises'
import { strictEqual } from 'assert'

let called = 0

async function * merge (a, b) {
  try {
    yield *a
    yield *b
  } finally {
    // Ensure that the two iterators are fully consumed
    // in case of streams, you might want to
    // destroy the stream itself.
    for await (const _ of a) {}
    for await (const _ of b) {}
  }
}

async function * funA (str) {
  try {
    yield str + 1
    await setTimeout(1000)
    yield str + 2
    await setTimeout(1000)
    yield str + 3
    await setTimeout(1000)
    yield str + 4
  } finally {
    called++
    console.log('finally', str)
  }
}

for await (const chunk of merge(funA('a'), funA('b'))) {
  console.log(chunk)
  // Comment the following line to see the difference
  if (chunk == 'a2') break
}

strictEqual(called, 2)

fail.mjs

import { setTimeout } from 'timers/promises'
import { strictEqual } from 'assert'

let called = 0

async function * merge (a, b) {
  yield *a
  yield *b
}

async function * funA (str) {
  try {
    yield str + 1
    await setTimeout(1000)
    yield str + 2
    await setTimeout(1000)
    yield str + 3
    await setTimeout(1000)
    yield str + 4
  } finally {
    called++    
    console.log('finally', str)
  }
}

for await (const chunk of merge(funA('a'), funA('b'))) {
  console.log(chunk)
  // Comment the following line to see the difference
  if (chunk == 'a2') break
}

strictEqual(called, 2)

return.mjs

import { setTimeout } from 'timers/promises'
import { strictEqual } from 'assert'

let called = 0

async function * merge (a, b) {
  try {
    yield *a
    yield *b
  } finally {
    const settled = await Promise.allSettled([
      a.next(),
      b.next(),
      a.return(),
      b.return()
    ])
    // For debugging purposes only
    console.log(settled)
  }
}

async function * funA (str) {
  try {
    yield str + 1
    await setTimeout(1000)
    yield str + 2
    await setTimeout(1000)
    yield str + 3
    await setTimeout(1000)
    yield str + 4
  } finally {
    called++    
    console.log('finally', str)
  }
}

for await (const chunk of merge(funA('a'), funA('b'))) {
  console.log(chunk)
  // Comment the following line to see the difference
  if (chunk == 'a2') break
}

strictEqual(called, 2)

throw.mjs

import { setTimeout } from 'timers/promises'
import { strictEqual } from 'assert'

let called = 0

async function * merge (a, b) {
  try {
    yield *a
    yield *b
  } finally {
    const settled = await Promise.allSettled([
      a.next(),
      b.next(),
      a.throw(new Error('kaboom a')),
      b.throw(new Error('kaboom b'))
    ])
    // For debugging purposes only
    console.log(settled)
  }
}

async function * funA (str) {
  try {
    yield str + 1
    await setTimeout(1000)
    yield str + 2
    await setTimeout(1000)
    yield str + 3
    await setTimeout(1000)
    yield str + 4
  } finally {
    called++
    console.log('finally', str)
  }
}

for await (const chunk of merge(funA('a'), funA('b'))) {
  console.log(chunk)
  // Comment the following line to see the difference
  if (chunk == 'a2') break
}

strictEqual(called, 2)