`setIntervalAsync`

Modern version of setInterval for promises and async functions available in Node.js and browsers.

setIntervalAsync works both on Node.js and in the browser, providing the same
familiar interface as setInterval for asynchronous functions, while preventing
multiple executions from overlapping in time.

Getting Started

Node.js

You can install setIntervalAsync using npm:

npm install set-interval-async
# If using Typescript, also run:
npm install -D @types/set-interval-async

Or using Yarn:

yarn add set-interval-async
# If using Typescript, also run:
yarn add -D @types/set-interval-async

Now, you can require setIntervalAsync in CommonJS:

// Choose one of the following flavors: dynamic, fixed, legacy.

const { setIntervalAsync } = require('set-interval-async/dynamic')
const { setIntervalAsync } = require('set-interval-async/fixed')
const { setIntervalAsync } = require('set-interval-async/legacy')
const { clearIntervalAsync } = require('set-interval-async')

// Or require all at once:

const {
  dynamic: { setIntervalAsync: setIntervalAsyncD },
  fixed: { setIntervalAsync: setIntervalAsyncF },
  legacy: { setIntervalAsync: setIntervalAsyncL },
  clearIntervalAsync
} = require('set-interval-async')

Or else, you can use ES6 modules syntax:

// Choose one of the following flavors: dynamic, fixed, legacy.

import { setIntervalAsync } from 'set-interval-async/dynamic'
import { setIntervalAsync } from 'set-interval-async/fixed'
import { setIntervalAsync } from 'set-interval-async/legacy'
import { clearIntervalAsync } from 'set-interval-async'

// Import all at once:

import {
  dynamic,
  fixed,
  legacy,
  clearIntervalAsync
} from 'set-interval-async'
const { setIntervalAsync: setIntervalAsyncD } = dynamic
const { setIntervalAsync: setIntervalAsyncF } = fixed
const { setIntervalAsync: setIntervalAsyncL } = legacy

Browser

In the browser, you can add a script tag in your HTML:

<script src="https://unpkg.com/set-interval-async"></script>

After the script is loaded, a module SetIntervalAsync will be defined in the global context. Now, you can retrieve the setIntervalAsync function in any of its flavors:

// Choose one of the following flavors: dynamic, fixed, legacy.

var setIntervalAsync = SetIntervalAsync.dynamic.setIntervalAsync
var setIntervalAsync = SetIntervalAsync.fixed.setIntervalAsync
var setIntervalAsync = SetIntervalAsync.legacy.setIntervalAsync

// Load `clearIntervalAsync` as well.

var clearIntervalAsync = SetIntervalAsync.clearIntervalAsync

Usage

In the most basic scenario, you can use setIntervalAsync the same way you would use vanilla setInterval. For example, the following code:

const {
  setIntervalAsync,
  clearIntervalAsync
} = require('set-interval-async/dynamic')

setIntervalAsync(
  () => console.log('Hello'),
  1000
)

will print 'Hello' to the console once every second. However, you can also provide an async function (or a function returning a promise):

const timer = setIntervalAsync(
  async () => {
    console.log('Hello')
    await doSomeWork()
    console.log('Bye')
  },
  1000
)

// or equivalently

const timer = setIntervalAsync(
  () => {
    console.log('Hello')
    return doSomeWork().then(
      () => console.log('Bye')
    )
  },
  1000
)

which has the added nicety that now you can wait until the cycle is fully stopped before moving on by using clearIntervalId. This is particularly useful when, at the end of a unit test, you want to make sure that no asynchronous code continues running by the time your test manager moves on to the next one.

it('should test something', async () => {
  const timer = setIntervalAsync(
    /* some async function */,
    /* some interval */
  )
  // Do some assertions.
  await clearIntervalAsync(timer)
  // At this point, all timers have been cleared, and the last
  // execution is guaranteed to have finished as well.
})

Where setIntervalAsync really shines is in those situations where the given asynchronous function might take longer to compute than the configured interval and, at the same time, is not safe to execute more than once at a time. Using vanilla setInterval will break your code in this scenario, whereas setIntervalAsync guarantees that the function will never execute more than once at the same time. For example, consider:

async function processQueue (queue) {
  if (queue.length === 0) {
    return
  }
  let head = queue[0]
  await doSomeWork(head)
  queue.shift() // Removes the first element.
}

The function above should never get called again before the previous execution is completed. Otherwise, the queue's first element will get processed twice, and the second element will be skipped. However, with setIntervalAsync, the following is perfectly safe:

setIntervalAsync(processQueue, 1000, queue)

since setIntervalAsync will guarantee that the function is never executed more than once at any given moment. You can choose whether you wish to use the Dynamic or Fixed flavors, which will either launch every execution as soon as possible or set a fixed delay between the end of one execution and the start of the next one. See Dynamic and Fixed setIntervalAsync for more details.

To see a full set of examples and instructions on how to run them, check out our examples directory.

A word of caution

While calling clearIntervalAsync to stop an interval is perfectly safe in any circumstance, please note that awaiting its result within the async handler itself will lead to undesirable results. For example, the code below:

const timer = setIntervalAsync(async () => {
  // ...
  if (shouldStop) {
    await clearIntervalAsync(timer);
    console.log('Stopped!');
  }
}, interval);

leads to a cyclical promise chain that will never be resolved (the console.log statement is unreachable). This is the case because:

await clearIntervalAsync(timer) will not resolve until the last execution has finished, AND
the last execution will not finish until await clearIntervalAsync(timer) has been resolved.

To prevent this cycle, always allow the async handler to complete without awaiting for the interval to be cleared. For example, by removing the await keyword entirely or by using an immediately-invoked function expression:

  if (shouldStop) {
    (async () => {
      await clearIntervalAsync(timer);
      console.log('Stopped!');
    })();
  }

Motivation

If you've ever had to deal with weird, subtle bugs as a consequence of using setInterval[1] on asynchronous functions, or had to manually reimplement setInterval using setTimeout[2] to prevent multiple executions of the same asynchronous function from overlapping, then this library is a drop-in replacement that will solve your issues.

setInterval runs a given function repeateadly, once every fixed number of milliseconds. This may cause problems whenever the function takes longer to execute than the given interval, since it will be called again before the first execution has finished. This is often a problem for non-reentrant functions; ie. functions that are not designed to allow multiple executions at the same time.

setIntervalAsync is a drop-in replacement of setInterval which shares the same API but is safe to use with non-reentrant, asynchronous functions.

[1] https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/setTimeout
[2] https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/setInterval

Dynamic and Fixed `setIntervalAsync`

setIntervalAsync provides two strategies which can be used to prevent a recurring function from executing more than once at any given moment:

Dynamic: If possible, the given function is called once every interval milliseconds. If any execution takes longer than the desired interval, the next execution is delayed until the previous one has finished, and called immediately after this condition is reached.
Fixed: The given function is called repeatedly, guaranteeing a fixed delay of interval milliseconds between the end of one execution and the start of the following one.

You can choose whichever strategy works best for your application. When in doubt, the Dynamic strategy will likely suffice for most use cases, keeping the interval as close as possible to the desired one.

Documentation

You can browse the full API in our Documentation page.

Contributing

In order to contribute to this project, you will need to first clone the repository:

git clone https://github.com/ealmansi/set-interval-async.git

Make sure that Yarn is installed globally on your system, install all project dependencies, and build the project:

yarn
yarn build

Now, you can run the tests and make sure that everything is up and running correctly:

yarn test

If the previous step succeeds, you're ready to start developing on this project.
Pull requests are welcome!

You can verify that your code follows the JavaScript Standard Style with the following command:

yarn lint

License

MIT

jamesst20/set-interval-async

setIntervalAsync