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JavaScript's promised convenience

2022-10-1812 minutes readJavaScript

The introduction of Promises in JavaScript was a great usability improvement when working with asynchronous code. They are usually much easier to comprehend compared to callbacks (see callback hell). It is not a new concept, though. Other languages have similar abstraction to handle asynchronous computations that eventually return a value: Rust has Futures, so does C++, C# has Tasks, and so do many other languages.

I will explore how the JavaScript implementation of Promise-chaining has an extra functionality that seems like a convenience, but proves difficult to work with in some cases, for example zx scripts. This makes Promise diverge from the monad definition.

Chaining Promises

Naturally, Promises can be chained using the then method.

function getRandomInteger() {
  return Math.round(Math.random() * 50);

function delay(ms: number): Promise<void> {
  return new Promise((resolve) => setTimeout(resolve, ms));

function getDelayedRandomInteger(): Promise<number> {
  return delay(1000).then(getRandomInteger);

// Print a random integer (multiplied by 2) after one second
  .then((integer) => integer * 2)

If we break it down into steps, it could look like so:

const randomIntegerPromise = getDelayedRandomInteger();
const doubledRandomIntegerPromise = randomIntegerPromise.then((value) => {
  return value * 2;

Looking at this syntax and ignoring other knowledge I have about the behavior of then, I can conclude that the callback provided to then will receive the value of the Promise it is attached to. Whatever the callback returns will be the value the final Promise resolves with.

To be concrete, randomIntegerPromise will resolve with some random integer, which will be passed as an argument (value) to the arrow function which returns the doubled value. That doubled value will be the value doubledRandomIntegerPromise resolves with.

All good so far. What if we wanted to chain another asynchronous operation? Let's use that random integer as a person ID and get that person details from the Star Wars API.

  .then((id) => fetch(`${id}`))
  .then((response) => response.json())

A point for you if you noticed we are using the same then method to chain promises but now the function returns asynchronous operations, aka Promises (both fetch and response.json() return Promises).

It still works, though. You can try it in the TypeScript playground.

What is convenient (or weird, depending on your preference) is that the type of myPromise in the following snippet is not Promise<Promise<Response>>, but just Promise<Response>.

const responsePromise: Promise<Response> = getDelayedRandomInteger().then(
  (id) => fetch(`${id}`)

then accepts functions that return both synchronous values and Promises. If the function returns a Promise, the outer Promise will resolve with that returned Promise value. If it returns a synchronous value, that value will be used as the final value.

It may seem a bit magical at first and seems convenient at first. It is not concerning as long as you are not actually interested in having access to the inner Promise from the outside. In other words, if you do not care about the final type actually being Promise<Promise<...>>, you can live a happy life appreciating the convenience of the overloaded then method. What if you need that inner promise?

Processes as promises in zx

zx is a great tool for writing scripts in JavaScript/TypeScript. I use it in favor of Bash scripts and appreciate how easy it is to read and refactor them.

When using zx, the primary way to run other processes is to use the $ tagged template literal. It returns a Promise that resolves with an object containing the output of the process.

import { $ } from "zx";

const echoProcess = $`echo "Hello world"`;
echoProcess.then(({ exitCode }) => console.log("Finished with", exitCode));
10:22 $ npx zx test.mjs
$ echo "Hello world"
Hello world
Finished with 0

Background tasks in zx

In some cases, we want to start a process and run it in the background while our zx script runs some other tasks. This happened to me when I wanted to start a postgres Docker container and keep it running in the background while the script leverages it later. Starting a Docker container takes a moment (especially when the image has to be pulled for the first time), so the script should wait for the container to be running before it continues.

The script could look like this (parts omitted for brevity):

const containerName = "postgres";

const waitForContainerToBeReady = () =>
  $`docker inspect -f '{{.State.Status}}' ${containerName}`.then(({ stdout }) =>
    stdout.trim() === "running" ? undefined : waitForContainerToBeReady()

const startPostgresContainer = () =>
  $`docker create --name ${containerName} ...`.then(() => {
    const postgresContainerProcess = $`docker start --attach ${containerName}`;

    return waitForContainerToBeReady().then(() => postgresContainerProcess);

const stopPostgresContainer = (postgresContainerProcess: ProcessPromise) =>

startPostgresContainer((postgresContainerProcess) => {
  // Run some SQL queries and then stop the process
  return $`docker exec ...`.then(() =>

The functions are neatly extracted so other scripts can use them easily and the code consists of higher-level blocks, which makes it easier to read.

There is a bug in this code which is strictly related to the overloaded behavior of then.

Unnecessary waiting

The bug is in the final then of the startPostgresContainer function.

const startPostgresContainer = () =>
  $`docker create --name ${containerName} ...`.then(() => {
    const postgresContainerProcess = $`docker start --attach ${containerName}`;

    return waitForContainerToBeReady().then(
      () =>
        // The bug is here 🐛

The intent is to return the entire postgresContainerProcess of type ProcessPromise (which is essentially Promise<ProcessOutput>) to the caller of startPostgresContainer. What happens instead is that then waits for the postgresContainerProcess to finish before resolving the outer promise returned from startPostgresContainer. This means we have a deadlock, because the postgres container will keep running in the background indefinitely and the zx script will wait for it to finish.

This is strictly related to the fact that then will wait for the inner Promise to resolve if it gets a Promise from the callback. It does not support returning the inner Promise as the resolved value of the outer Promise.


There are 2 workarounds I found to this problem:

  1. Separate creating processes from doing additional operations on them.

    This means a function like startPostgresContainer cannot both start the container and wait for it to be ready. It should only create the Promise and pass it to the caller. Then, the caller always gets the full Promise and can await it whenever they want. This means the calling code would have to be refactored like so:

    startPostgresContainer((postgresContainerProcess) =>
        .then(() => $`docker exec ...`)
        .then(() => stopPostgresContainer(postgresContainerProcess))
  2. Wrap the inner Promise in an object or array to prevent then waiting for it.

    This is explicitly avoiding the convenience of then by wrapping the inner Promise in some container (object/array), so then returns that container as-is rather than waiting for it to resolve before continuing.

    const startPostgresContainer = () =>
      $`docker create --name ${containerName} ...`.then(() => {
        const postgresContainerProcess = $`docker start --attach ${containerName}`;
        return waitForContainerToBeReady().then(() =>
          // Notice we return an array now
        // We unwrap that array in the consuming code
      ) =>
        $`docker exec ...`.then(() =>

    A more contrived example:

    import { $ } from "zx";
    const workflow = async () => {
      const sleep = $`sh -c "sleep 10 && echo Awoken from sleep"`;
      await $`echo "Hello world"`;
      return [sleep];
    const [sleep] = await workflow();
    console.log("Killing sleeping process");
    await sleep.nothrow().kill();


    11:06 $ npx zx test.mjs
    $ sh -c "sleep 10 && echo Awoken from sleep"
    $ echo "Hello world"
    Hello world
    Killing sleeping process

I like the second solution better. It keeps the waitForContainerToBeReady logic inside of startPostgresContainer while being only slightly less convenient for the caller thanks to array destructuring.

async/await is the same

In case you were wondering, using async/await instead of chaining .then behaves the same way. If we rewrote startPostgresContainer to use async/await, the bug would still be there:

const startPostgresContainer = async () =>
  await $`docker create --name ${containerName} ...`
    const postgresContainerProcess = $`docker start --attach ${containerName}`;

    await waitForContainerToBeReady();
    return postgresContainerProcess;

The Promise returned from startPostgresContainer would resolve when postgresContainerProcess resolves. This means, startPostgresContainer would resolve after the container process finishes.

Awaiting problems in JavaScript goes over this in more detail.

Monadic chaining

In functional programming terminology, I would characterize Promise as a slightly skewed monad. The overloaded then is the reason why it is slightly skewed.

Monads have 2 separate methods to transform them:

  • map(callback) which makes the outer monad contain whatever value the inner function returns. This is like then when the callback returns a regular value.
  • chain(callback) (also known as flatMap or bind) which requires that the callback returns a monad, and the outer monad becomes that inner monad. This is like then when the callback returns another Promise.

What is nice in having 2 separate methods is that the programmer gets to choose which one is needed at which point. Even if you decide to use the other one in the middle of the function, changing the name from one to the other is simple enough.

then does not let you choose - the behavior is chosen for you based on the value you return. If you want the map functionality and want to return a Promise, you are required to do workarounds.

If you are using fp-ts, you are in luck. Its Task module contains separate chain and map functions. If you are using regular Promises, you are stuck doing workarounds.


Promise then API was designed in a way that differs from the usual functional programming monad API. It offers more convenience in most cases, but makes more advanced cases require workarounds. I appreciate precision and I would love if the Promise API had 2 different methods for chain and map. I doubt the core API will change, though, so this thought will stay an unfulfilled dream.

Read more how this behavior translates to async/await and what problems lie there.