p-wait-for

What is p-wait-for?

The p-wait-for npm package is a utility that allows you to wait for a condition to be met before proceeding with the execution of your code. It is particularly useful for scenarios where you need to poll for a certain state or condition to be true before continuing.

What are p-wait-for's main functionalities?

Basic Usage

This feature allows you to wait for a basic asynchronous condition to be met. The `pWaitFor` function takes a condition function that returns a promise, and it resolves when the condition returns true.

const pWaitFor = require('p-wait-for');

const condition = async () => {
  // Some asynchronous condition
  return await someAsyncCheck();
};

(async () => {
  await pWaitFor(condition);
  console.log('Condition met!');
})();

Timeout

This feature allows you to specify a timeout for the condition to be met. If the condition is not met within the specified timeout, an error is thrown.

const pWaitFor = require('p-wait-for');

const condition = async () => {
  // Some asynchronous condition
  return await someAsyncCheck();
};

(async () => {
  try {
    await pWaitFor(condition, { timeout: 5000 });
    console.log('Condition met within timeout!');
  } catch (error) {
    console.error('Condition not met within timeout');
  }
})();

Interval

This feature allows you to specify an interval at which the condition function is called. This can be useful to reduce the frequency of checks and avoid overwhelming the system.

const pWaitFor = require('p-wait-for');

const condition = async () => {
  // Some asynchronous condition
  return await someAsyncCheck();
};

(async () => {
  await pWaitFor(condition, { interval: 1000 });
  console.log('Condition met!');
})();

Other packages similar to p-wait-for

await-poll

The await-poll package provides similar functionality by allowing you to poll for a condition to be met. It offers a simple API for waiting for a condition with customizable intervals and timeouts. Compared to p-wait-for, await-poll has a more straightforward API but may lack some of the advanced options.

promise-poller

The promise-poller package is another alternative that allows you to poll for a condition to be met. It provides more advanced options such as retries, delays, and custom error handling. Compared to p-wait-for, promise-poller offers more flexibility and customization options.

README

p-wait-for

Wait for a condition to be true

Can be useful for polling.

Install

npm install p-wait-for

Usage

import pWaitFor from 'p-wait-for';
import {pathExists} from 'path-exists';

await pWaitFor(() => pathExists('unicorn.png'));
console.log('Yay! The file now exists.');

API

pWaitFor(condition, options?)

Returns a Promise that resolves when condition returns true. Rejects if condition throws or returns a Promise that rejects.

condition

Type: Function

Expected to return Promise<boolean> | boolean.

options

Type: object

interval

Type: number
Default: 20

Number of milliseconds to wait after condition resolves to false before calling it again.

timeout

Type: number | TimeoutOptions
Default: Infinity

Number of milliseconds to wait before automatically rejecting with a TimeoutError.

You can customize the timeout Error by specifying TimeoutOptions.

import pWaitFor from 'p-wait-for';
import {pathExists} from 'path-exists';

const originalSetTimeout = setTimeout;
const originalClearTimeout = clearTimeout;

sinon.useFakeTimers();

await pWaitFor(() => pathExists('unicorn.png'), {
	timeout: {
		milliseconds: 100,
		message: new MyError('Time’s up!'),
		customTimers: {
			setTimeout: originalSetTimeout,
			clearTimeout: originalClearTimeout
		}
	}
});

console.log('Yay! The file now exists.');

milliseconds

Type: number
Default: Infinity

Milliseconds before timing out.

Passing Infinity will cause it to never time out.

message

Type: string | Error Default: 'Promise timed out after 50 milliseconds'

Specify a custom error message or error.

If you do a custom error, it's recommended to sub-class TimeoutError.

customTimers

Type: object with function properties setTimeout and clearTimeout

Custom implementations for the setTimeout and clearTimeout functions.

Useful for testing purposes, in particular to work around sinon.useFakeTimers().

fallback

Type: Function

Do something other than rejecting with an error on timeout.

Example:

import pWaitFor from 'p-wait-for';
import {pathExists} from 'path-exists';

await pWaitFor(() => pathExists('unicorn.png'), {
	timeout: {
		milliseconds: 50,
		fallback: () => {
			console.log('Time’s up! executed the fallback function!');
		},
	}
});

before

Type: boolean
Default: true

Whether to run the check immediately rather than starting by waiting interval milliseconds.

Useful for when the check, if run immediately, would likely return false. In this scenario, set before to false.

resolveWith(value)

Resolve the main promise with a custom value.

import pWaitFor from 'p-wait-for';
import pathExists from 'path-exists';

const path = await pWaitFor(async () => {
	const path = getPath();
	return await pathExists(path) && pWaitFor.resolveWith(path);
});

console.log(path);

TimeoutError

Exposed for instance checking.

Related

• p-whilst - Calls a function repeatedly while a condition returns true and then resolves the promise
• More…