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poll

A simple poll function based on async, await, and an infinite loop.

Links:

Features:

  • Customizable delay via callback function (e.g. to implement exponential backoff)
  • Stop polling programmatically (e.g. stop polling once a certain condition is fulfilled)

Contents

Installation & usage

As npm package

Install the poll package.

npm install poll

Import the poll function and use it.

import { poll } from 'poll'

function fn() {
	console.log('Hello, beautiful!')
}

poll(fn, 1000)

As plain JS file

Download the poll module.

curl -O 'https://cdn.jsdelivr.net/npm/poll@latest/dist/poll.js'

Import the poll function and use it.

<script type="module">
	import { poll } from './poll.js'

	function fn() {
		console.log('Hello, beautiful!')
	}

	poll(fn, 1000)
</script>

Documentation

Basic usage of poll looks like this:

poll(function () {
	console.log('Hello, beautiful!')
}, 1000)

Parameters

fn

Type: () => any

A function to be called every delay milliseconds. No parameters are passed to fn upon calling it.

delayOrDelayCallback

Type: number | (() => number)

The delay (in milliseconds) to wait before calling the function fn again. If a function is provided instead of a number, it is evaluated during every polling cycle right before the wait period. If the delay is a negative number, zero will be used instead.

shouldStopPolling (optional)

Type: () => boolean | Promise<boolean>
Default: () => false

A function returning a boolean (or a function returning a promise resolving to a boolean) indicating whether to stop the polling process. The shouldStopPolling callback function is called twice during one polling cycle:

  • After the result of the call to fn was successfully awaited (right before triggering a new delay period).
  • After the delay has passed (right before calling fn again).

This guarantees two things:

  • A currently active execution of fn will be completed.
  • No new calls to fn will be triggered.

Return value

None.

Examples

Minimal

The poll function expects two parameters: A callback function and a delay. After calling poll with these parameters, the callback function will be called. After it’s done being executed, the poll function will wait for the specified delay. After the delay, the process starts from the beginning.

const pollDelayInMinutes = 10

async function getStatusUpdates() {
	const pokemonId = Math.floor(Math.random() * 151 + 1)
	const response = await fetch(`https://pokeapi.co/api/v2/pokemon/${pokemonId}/`)
	const pokemon = await response.json()
	console.log(pokemon.name)
}

poll(getStatusUpdates, pollDelayInMinutes * 60 * 1000)

Note that poll will not cause a second call to the callback function if the first call is never finishing. For example, if the endpoint /status does not respond and the server doesn’t time out the connection, poll will still be waiting for the callback function to resolve until the dusk of time.

Stop polling

You can pass a callback function to poll for its third parameter. It’s evaluated before and after calls to the polled function. If it evaluates to a truthy value, the poll function’s loop will stop and the function returns.

let stopPolling = false

function fn() {
	console.log('Hello, beautiful!')
}

setTimeout(() => {
	stopPolling = true
}, 1000)

poll(fn, 50, () => stopPolling)

In this example, the shouldStopPolling callback function evaluates to true after the setTimeout function causes stopPolling to be set to true after 1000 milliseconds. The next time shouldStopPolling is evaluated, it will cause poll to exit normally.

Stop polling using asynchronous shouldStopPolling function

You can also provide an asynchronous function for the shouldStopPolling callback function.

let stopPolling = false
const shouldStopPolling = () => new Promise((resolve) => {
	setTimeout(() => {
		resolve(stopPolling)
	}, 100)
})

function fn() {
	console.log('Hello, beautiful!')
}

setTimeout(() => {
	stopPolling = true
}, 1000)

poll(fn, 50, shouldStopPolling)

Beware that this function will be called twice per polling cycle.

Exponential backoff: increase polling interval with every cycle

By providing a function that returns the delay value instead of the delay value itself, you can customize the behavior of the polling interval. In the following example, the delay doubles with each polling cycle.

const pollDelayInMinutes = 1
let delay = pollDelayInMinutes * 60 * 1000

const startTime = Date.now()

async function getStatusUpdates() {
	const pokemonId = Math.floor(Math.random() * 151 + 1)
	const response = await fetch(`https://pokeapi.co/api/v2/pokemon/${pokemonId}/`)
	const pokemon = await response.json()
	const seconds = (Date.now() - startTime) / 1000
	console.log('Seconds passed:', seconds, pokemon.name)
}

const delayCallback = () => {
	const currentDelay = delay
	delay *= 2
	return currentDelay
}

poll(getStatusUpdates, delayCallback)

Versioning

This package uses semantic versioning.

Update package version

  1. Make some changes and run the tests and the build script.

    npm test
npm run build

  2. Commit the changes.

  3. Verify that you’re authenticated with npm.

    npm whomai

    If you’re not authenticated, do so using npm login.

  4. Change the package’s version locally.

    # See `npm version --help` for more options
npm version minor

    This changes the version number in the package.json file and adds a new git tag matching the new version.

  5. Push your changes and the updated git tags separately.

    git push
git push --tags

  6. Publish the package.

    npm publish