PaxJS Promise

Global Promise object improvements (zero-dependency, 1Kb).

Why

There is a great library bluebird, which provides a similar functionality, but often you need only part of it and it's much easier to use it as global Promise object methods, than import it in every module where you need it.

paxjs-promise works everywhere where global Promise object is available. In Internet Explorer it does nothing.

Installation

Open a Terminal in your project's folder and run,

npm install paxjs-promise

Usage

Import paxjs-promise at start of your app entry point module. After that global Promise object will be enhanced and its new methods will be available in all your code.

import 'paxjs-promise'

or

require('paxjs-promise')

If you need only part of paxjs-promise functionality, you can use it like this, without using the whole library.

import 'paxjs-promise/delay'
import 'paxjs-promise/time'

Promise.delay

The same as bluebird Promise.delay

Promise.delay(
  int ms,
  [any|Promise<any> value=undefined]
) -> Promise

Returns a promise that will be resolved with value (or undefined) after given ms milliseconds. If value is a promise, the delay will start counting down when it is fulfilled and the returned promise will be fulfilled with the fulfillment value of the value promise. If value is a rejected promise, the resulting promise will be rejected immediately.

Use cases

It's often in your app UX you have states that are limited in time (e.g., request success and failure feedbacks). Promise.delay gives you short and neat syntax how to handle such cases.

// raw promises

_onSubmit() {
  createEntity(this.state.credentials)
    .then(showSuccessMessage)
    .catch(showErrorMessage)
    .delay(1000)
    .then(hideMessage)
}

// async/await

async _onSubmit() {
  try {
    await createEntity(this.state.credentials)
 
    showSuccessMessage()
  } catch(err) {
    showErrorMessage()
  } finally {
    await Promise.delay(1000)
 
    hideMessage()
  }
}

Another case is when you have additional request after your main request and you don't want to overload the app after your main request is finished to not slow down its feedback rendering. You can do like this

// raw promises

_onButtonClick() {
  buyProduct(this.state.productId)
    .then(showSuccessMessage)
    .delay(1000)
    .then(sendAnaylitics)
}

// async/await

async _onButtonClick() {
  try {
    await buyProduct(this.state.productId)
 
    showSuccessMessage()
 
    await Promise.delay(1000)
 
    sendAnaylitics()
  } catch(err) {
    ...
  }
}

You can use Promise.delay to execute code in next tick.

async _onClick() {
  this.setState({ message: 'Hello World!' })
 
  await Promise.delay(0)
 
  console.log(this.state.message) // 'Hello World!'
}

is equivalent of

_onClick() {
  this.setState({ message: 'Hello World!' })
 
  setTimeout(() => {
    console.log(this.state.message) // 'Hello World!'
  }, 0)
}

When you call Promise.delay without ms argument, it falls to just Promise.resolve

You can pass value or another promise as a second argument in Promise.delay.

function mockCheckPermissions() {
  return Promise.delay(1000, { status: 200, message: 'OK' })
}

Promise.prototype.time

Promise.prototype.time(
  [
    Object {
      min: int,
      max: int,
      timeoutMessage: String="operation timed out"
    } options
  ]
) -> Promise

Adds min and max time to promise. If min time is specified, the resulting promise will be resolved not before given milliseconds. If min time is specified and promise is rejected, the resulting promise will be rejected immediately. If max time is specified and promise takes more time to resolve, the resulting promise will be rejected with Promise.TimeError. max can't be less than min, else the resulting promise will be rejected immediately with Promise.TimeError

Use cases

In UX you often need to set minimum and maximum time for "in-progress" state (e.g., 300ms and 3000ms). With Promise.prototype.time you can do it like this

async _onLoginButtonClick() {
  try {
    showInProgressState()
 
    await authenticate(this.state.credentials)
      .time({ min: 300, max: 3000 })
 
    showSuccessState()
  } catch(err) {
    console.log('err', err)
 
    if (err instanceof Promise.TimeError) {
      promptToTryAgain()
    } else {
      showErrorMessage()
    }
  }
 
}

In Node.js sometimes you need to limit maximum time for request to external service

app.get('/user/:id/weather', function(req, res, next) {
  getUserCity(req.params.id)
    .then(city =>
      getCityWeather(city).time({
        max: 1000,
        timeoutMessage: 'Request to external service timed out.',
      })
    )
    .then(result => res.status(200).json(result))
    .catch(err => res.status(500).send(err))
    .then(next)
})