promise-assist

promise-assist

Several helper functions when working with native promises.

API

sleep

Useful for waiting a specific amount of time before continuing an operation.

import { sleep } from 'promise-assist'

async function myOperation() {
    const startTime = Date.now()
    await sleep(500)
    console.log(`${Date.now() - startTime}ms passed!`)
}

timeout

Useful for limiting the amount of time an async Promise-based operation can take.

import { timeout } from 'promise-assist'

async function myOperation() {
    try {
        const data = await timeout(
            fetchDataFromServer(), // pass a Promise to the timeout function
            10000, // request will be limited to 10 seconds
            `failed loading required data from backend`
        )
        // do something with the data
    } catch (e) {
        // handle errors
    }
}

deferred

Creates a deferred Promise, where resolve/reject are exposed to the place that holds the promise.

Generally a bad practice, but there are use-cases, such as mixing callback-based and Promise-based APIs, where this is helpful.

import { deferred } from 'promise-assist'

const { promise, resolve, reject } = deferred<string>()

// `resolve` or `reject` calls are reflected on `promise`
promise.then(value => console.log(value))
resolve('some text')
// 'some text' is printed to console

retry

Executes provided action (sync or async) and returns its value. If action throws or rejects, it will retry execution several times before failing.

Defaults are:

• 3 retries
• no delay between retries
• no timeout to stop trying

These can be customized via a second optional options parameter.

import { retry } from 'promise-assist'

// with default options
retry(() => fetch('http://some-url/asset.json'))
    .then(value => value.json())
    .then(console.log)
    .catch(e => console.error(e))

// with custom options
retry(() => fetch('http://some-url/asset.json'), {
        retries: Infinity,         // infinite number of retries
        delay: 10 * 1000,          // 10 seconds delay between retries
        timeout: 2 * 60 * 1000     // 2 minutes timeout to stop trying
    })
    .then(value => value.json())
    .then(console.log)
    .catch(e => console.error(e))

waitFor

Same as retry, but with defaults that make more sense for tests:

• delay: 10
• timeout: 500
• retries: Infinity

It can be used to wait for some assertion to pass.

import { waitFor } from 'promise-assist'

describe('suit', () => {
    it('should wait for an assertion to pass', async () => {
        let trueLater = false
        setTimeout(() => { trueLater = true }, 50)

        await waitFor(() => {
            expect(trueLater).to.equal(true)
        })
    })
})

License

MIT