await-timeout

await-timeout

A Promise-based API for setTimeout / clearTimeout

Contents

• Installation
• Usage
• API
  • new Timeout()
  • .set()
  • .wrap()
  • .clear()
  • .id
  • .delay
• Motivation
• Related resources
• License

Installation

npm install await-timeout --save

Usage

1. Just wait some time:

   import Timeout from 'await-timeout';
   
   // wait 1000 ms and resolve
   await Timeout.set(1000);
   
   // wait 1000 ms and reject with 'Timeout!'
   await Timeout.set(1000, 'Timeout!');
   

2. Use Timeout instance inside try...finally block to make proper cleanup:

   import Timeout from 'await-timeout';
   
   const timer = new Timeout();
   try {
     await Promise.race([
       fetch('https://example.com'),
       timer.set(1000, 'Timeout!')
     ]);
   } finally {
     timer.clear();
   }
   

   Without a timer cleanup you may get unexpected effects in you code - as all promises in Promise.race are get fulfilled.

API

new Timeout()

Constructs new timeout instance. It does not start timer but creates variable for timer manipulation.

const timer = new Timeout();

Note: having separate timer variable is useful for clearing timeout in finally block

.set(delay, [rejectReason]) ⇒ Promise

Starts new timer like setTimeout() and returns promise. The promise will be resolved after delay milliseconds:

const timer = new Timeout();
timer.set(1000)
  .then(() => console.log('1000 ms passed.'));

If you provide rejectReason - a timer promise will be rejected with specified reason:

// rejects with Error: Timeout after 1000 ms:
timer.set(1000, 'Timeout after 1000 ms');
  
// above is actually shortcut for:
timer.set(1000).then(() => Promise.reject(new Error('Timeout after 1000 ms')));  

If you need to just wait some time - use static version of .set():

await Timeout.set(1000);

.wrap(promise, delay, [rejectReason]) ⇒ Promise

Wraps existing promise with timeout:

• returned promise automatically rejected after timeout
• timeout automatically cleared if main promise resolves first

async function fetchWithTimeout() {
  const promise = fetch('https://example.com');
  return Timeout.wrap(promise, 1000, 'Timeout');
}

Actually it is a shortcut for:

async function fetchWithTimeout() {
    const timer = new Timeout();
    try {
      const promise = fetch('https://example.com');
      return await Promise.race([
        promise,
        timer.set(1000, 'Timeout')
      ]);
    } finally {
      timer.clear();
    }
}

.clear()

Clears existing timeout like clearTimeout().

const timer = new Timeout();
timer.set(1000)
  .then(() => console.log('This will never be called, because timeout is cleared on the next line'));
timer.clear();

With ES7 async / await .clear() can be used in finally block:

async function foo() {
  const timer = new Timeout();
  try {
    // some async stuff
  } finally {
    timer.clear();
  }
}

.id ⇒ ?Number|?Timeout

Returns result of setTimeout call. That is Number timeout id in browser and Timeout instance in Node.js.

.delay ⇒ ?Number

Returns last delay value used. Delay is useful for generating reject reason:

const timer = new Timeout();
timer.set(1000, () => new Error(`Timeout: ${timer.delay}`));

Motivation

Before making this library I've researched several similar packages on Npm. But no one satisfied all my needs together:

1. Convenient way to cancel timeout. I typically use it with Promise.race() and don't want timer to trigger if main promise is resolved first.
2. API similar to setTimeout / clearTimeout. I get used to these functions and would like to have mirror syntax.
3. Easy rejection of timeout promise. Passing error message should be enough.
4. No monkey-patching of Promise object.
5. Zero dependencies.

Related resources

• The right way to clear timeout in Promise.race()
• Applying a timeout to your promises
• How to make a promise from setTimeout
• Is there a version of setTimeout that returns an ES6 promise?

License

MIT @ Vitaliy Potapov