abort-controller-x

Abort Controller Extras

Abortable async function primitives and combinators.

• Installation
• Abort Controller
• Abortable Functions
• Composing Abortable Functions
• Companion Packages
• API
  • all
  • race
  • delay
  • waitForEvent
  • forever
  • spawn
  • retry
  • execute
  • abortable
  • run
  • AbortError
  • isAbortError
  • throwIfAborted
  • rethrowAbortError
  • catchAbortError

Installation

yarn add abort-controller-x

Abort Controller

See AbortController MDN page. AbortController is available in NodeJS, a polyfill is available for older versions.

Abortable Functions

We define abortable function as a function that obeys following rules:

• It must accept AbortSignal in its arguments.
• It must return a Promise.
• It must add abort event listener to the AbortSignal. Once the AbortSignal is aborted, the returned Promise must reject with AbortError either immediately, or after doing any async cleanup. It's also possible to reject with other errors that happen during cleanup.
• Once the returned Promise is fulfilled or rejected, it must remove abort event listener.

An example of abortable function is the standard fetch function.

Composing Abortable Functions

This library provides a way to build complex abortable functions using standard async/await syntax, without the burden of manually managing abort event listeners. You can reuse a single AbortSignal between many operations inside a parent function:

/**
 * Make requests repeatedly with a delay between consecutive requests
 */
async function makeRequests(signal: AbortSignal): Promise<never> {
  while (true) {
    await fetch('...', {signal});
    await delay(signal, 1000);
  }
}

const abortController = new AbortController();

makeRequests(abortController.signal).catch(catchAbortError);

process.on('SIGTERM', () => {
  abortController.abort();
});

The above example can be rewritten in a more ergonomic way using run helper.

Usually you should only create AbortController somewhere on the top level, and in regular code use async/await and pass AbortSignal to abortable functions provided by this library or custom ones composed of other abortable functions.

Companion Packages

• abort-controller-x-rxjs — Abortable helpers for RxJS.

API

all

function all<T>(
  signal: AbortSignal,
  executor: (innerSignal: AbortSignal) => readonly PromiseLike<T>[],
): Promise<T[]>;

Abortable version of Promise.all.

Creates new inner AbortSignal and passes it to executor. That signal is aborted when signal is aborted or any of the promises returned from executor are rejected.

Returns a promise that fulfills with an array of results when all of the promises returned from executor fulfill, rejects when any of the promises returned from executor are rejected, and rejects with AbortError when signal is aborted.

The promises returned from executor must be abortable, i.e. once innerSignal is aborted, they must reject with AbortError either immediately, or after doing any async cleanup.

Example:

const [result1, result2] = await all(signal, signal => [
  makeRequest(signal, params1),
  makeRequest(signal, params2),
]);

race

function race<T>(
  signal: AbortSignal,
  executor: (innerSignal: AbortSignal) => readonly PromiseLike<T>[],
): Promise<T>;

Abortable version of Promise.race.

Creates new inner AbortSignal and passes it to executor. That signal is aborted when signal is aborted or any of the promises returned from executor are fulfilled or rejected.

Returns a promise that fulfills or rejects when any of the promises returned from executor are fulfilled or rejected, and rejects with AbortError when signal is aborted.

The promises returned from executor must be abortable, i.e. once innerSignal is aborted, they must reject with AbortError either immediately, or after doing any async cleanup.

Example:

const result = await race(signal, signal => [
  delay(signal, 1000).then(() => ({status: 'timeout'})),
  makeRequest(signal, params).then(value => ({status: 'success', value})),
]);

if (result.status === 'timeout') {
  // request timed out
} else {
  const response = result.value;
}

delay

function delay(signal: AbortSignal, dueTime: number | Date): Promise<void>;

Return a promise that resolves after delay and rejects with AbortError once signal is aborted.

The delay time is specified as a Date object or as an integer denoting milliseconds to wait.

Example:

// Make a request repeatedly with a delay between consecutive requests
while (true) {
  await makeRequest(signal, params);
  await delay(signal, 1000);
}

Example:

// Make a request repeatedly with a fixed interval
import {addMilliseconds} from 'date-fns';

let date = new Date();

while (true) {
  await makeRequest(signal, params);

  date = addMilliseconds(date, 1000);
  await delay(signal, date);
}

waitForEvent

function waitForEvent<T>(
  signal: AbortSignal,
  target: EventTargetLike<T>,
  eventName: string,
  options?: EventListenerOptions,
): Promise<T>;

Returns a promise that fulfills when an event of specific type is emitted from given event target and rejects with AbortError once signal is aborted.

Example:

// Create a WebSocket and wait for connection
const webSocket = new WebSocket(url);

const openEvent = await race(signal, signal => [
  waitForEvent<WebSocketEventMap['open']>(signal, webSocket, 'open'),
  waitForEvent<WebSocketEventMap['close']>(signal, webSocket, 'close').then(
    event => {
      throw new Error(`Failed to connect to ${url}: ${event.reason}`);
    },
  ),
]);

forever

function forever(signal: AbortSignal): Promise<never>;

Return a promise that never fulfills and only rejects with AbortError once signal is aborted.

spawn

function spawn<T>(
  signal: AbortSignal,
  fn: (signal: AbortSignal, effects: SpawnEffects) => Promise<T>,
): Promise<T>;

type SpawnEffects = {
  defer(fn: () => void | Promise<void>): void;
  fork<T>(fn: (signal: AbortSignal) => Promise<T>): ForkTask<T>;
};

type ForkTask<T> = {
  abort(): void;
  join(): Promise<T>;
};

Run an abortable function with fork and defer effects attached to it.

spawn allows to write Go-style coroutines.

• SpawnEffects.defer

  Schedules a function to run after spawned function finishes.

  Deferred functions run serially in last-in-first-out order.

  Promise returned from spawn resolves or rejects only after all deferred functions finish.

• SpawnEffects.fork

  Executes an abortable function in background.

  If a forked function throws an exception, spawned function and other forks are aborted and promise returned from spawn rejects with that exception.

  When spawned function finishes, all forks are aborted.

• ForkTask.abort

  Abort a forked function.

• ForkTask.join

  Returns a promise returned from a forked function.

Example:

// Connect to a database, then start a server, then block until abort.
// On abort, gracefully shutdown the server, and once done, disconnect
// from the database.
spawn(signal, async (signal, {defer}) => {
  const db = await connectToDb();

  defer(async () => {
    await db.close();
  });

  const server = await startServer(db);

  defer(async () => {
    await server.close();
  });

  await forever(signal);
});

Example:

// Connect to a database, then start an infinite polling loop.
// On abort, disconnect from the database.
spawn(signal, async (signal, {defer}) => {
  const db = await connectToDb();

  defer(async () => {
    await db.close();
  });

  while (true) {
    await poll(signal, db);
    await delay(signal, 5000);
  }
});

Example:

// Acquire a lock and execute a function.
// Extend the lock while the function is running.
// Once the function finishes or the signal is aborted, stop extending
// the lock and release it.
import Redlock = require('redlock');

const lockTtl = 30_000;

function withLock<T>(
  signal: AbortSignal,
  redlock: Redlock,
  key: string,
  fn: (signal: AbortSignal) => Promise<T>,
): Promise<T> {
  return spawn(signal, async (signal, {fork, defer}) => {
    const lock = await redlock.lock(key, lockTtl);

    defer(() => lock.unlock());

    fork(async signal => {
      while (true) {
        await delay(signal, lockTtl / 10);
        await lock.extend(lockTtl);
      }
    });

    return await fn(signal);
  });
}

const redlock = new Redlock([redis], {
  retryCount: -1,
});

await withLock(signal, redlock, 'the-lock-key', async signal => {
  // ...
});

retry

function retry<T>(
  signal: AbortSignal,
  fn: (signal: AbortSignal, attempt: number, reset: () => void) => Promise<T>,
  options?: RetryOptions,
): Promise<T>;

type RetryOptions = {
  baseMs?: number;
  maxDelayMs?: number;
  maxAttempts?: number;
  onError?: (error: unknown, attempt: number, delayMs: number) => void;
};

Retry a function with exponential backoff.

• fn

  A function that will be called and retried in case of error. It receives:

  • signal

    AbortSignal that is aborted when the signal passed to retry is aborted.

  • attempt

    Attempt number starting with 0.

  • reset

    Function that sets attempt number to -1 so that the next attempt will be made without delay.

• RetryOptions.baseMs

  Starting delay before first retry attempt in milliseconds.

  Defaults to 1000.

  Example: if baseMs is 100, then retries will be attempted in 100ms, 200ms, 400ms etc (not counting jitter).

• RetryOptions.maxDelayMs

  Maximum delay between attempts in milliseconds.

  Defaults to 15 seconds.

  Example: if baseMs is 1000 and maxDelayMs is 3000, then retries will be attempted in 1000ms, 2000ms, 3000ms, 3000ms etc (not counting jitter).

• RetryOptions.maxAttempts

  Maximum for the total number of attempts.

  Defaults to Infinity.

• RetryOptions.onError

  Called after each failed attempt before setting delay timer.

  Rethrow error from this callback to prevent further retries.

execute

function execute<T>(
  signal: AbortSignal,
  executor: (
    resolve: (value: T) => void,
    reject: (reason?: any) => void,
  ) => () => void | PromiseLike<void>,
): Promise<T>;

Similar to new Promise(executor), but allows executor to return abort callback that is called once signal is aborted.

Returned promise rejects with AbortError once signal is aborted.

Callback can return a promise, e.g. for doing any async cleanup. In this case, the promise returned from execute rejects with AbortError after that promise fulfills.

abortable

function abortable<T>(signal: AbortSignal, promise: PromiseLike<T>): Promise<T>;

Wrap a promise to reject with AbortError once signal is aborted.

Useful to wrap non-abortable promises. Note that underlying process will NOT be aborted.

run

function run(fn: (signal: AbortSignal) => Promise<void>): () => Promise<void>;

Invokes an abortable function with implicitly created AbortSignal.

Returns a function that aborts that signal and waits until passed function finishes.

Any error other than AbortError thrown from passed function will result in unhandled promise rejection.

Example:

const stop = run(async signal => {
  try {
    while (true) {
      await delay(signal, 1000);
      console.log('tick');
    }
  } finally {
    await doCleanup();
  }
});

// abort and wait until cleanup is done
await stop();

This function is also useful with React useEffect hook:

// make requests periodically while the component is mounted
useEffect(
  () =>
    run(async signal => {
      while (true) {
        await makeRequest(signal);
        await delay(signal, 1000);
      }
    }),
  [],
);

AbortError

class AbortError extends Error

Thrown when an abortable function was aborted.

Warning: do not use instanceof with this class. Instead, use isAbortError function.

isAbortError

function isAbortError(error: unknown): boolean;

Checks whether given error is an AbortError.

throwIfAborted

function throwIfAborted(signal: AbortSignal): void;

If signal is aborted, throws AbortError. Otherwise does nothing.

rethrowAbortError

function rethrowAbortError(error: unknown): void;

If error is AbortError, throws it. Otherwise does nothing.

Useful for try/catch blocks around abortable code:

try {
  await somethingAbortable(signal);
} catch (err) {
  rethrowAbortError(err);

  // do normal error handling
}

catchAbortError

function catchAbortError(error: unknown): void;

If error is AbortError, does nothing. Otherwise throws it.

Useful for invoking top-level abortable functions:

somethingAbortable(signal).catch(catchAbortError);

Without catchAbortError, aborting would result in unhandled promise rejection.