successive ("that's excessive!")

npm i -S successive

import { after } from 'successive';  // client or server

after - the async utility you didn't know you needed.

after is the easiest way to create an Observable of a value, or a function call, after any amount of delay.

It returns an object with a dual identity as both a Promise and an Observable: an RxJS Observable underneath, but exposes a then method, so it can be awaited, or chained like a Promise.

Here are some things you can create with it:

await after(1000)                 // A promise-like duration

after(1000, {data: {}})           // A mock endpoint return value

after(15*60*1000, logout)         // A deferred timer until a function call (an Observable)
   .subscribe()                   //  - Begin the timer
   .unsubscribe()                 //  - Stop the timer (logout will not be called)

concat(                           // A mock user-behavior (cancelable!)
  after(1000, () => fillEmail()),
  after(1000, () => fillPassword())
)

Overview

Allows you to:

  • Mix and match Promises and Observables.
  • Support cancelation without additional objects (AbortController, clearTimeout)
  • Create scripts with specific delays to test debouncing, etc.
  • Compose in a cancelation-preserving way

Example Use Cases

after is a Promise (a "thenable")

When used as you would a Promise, after() represents a delay, an value, or a value calculated after either a) a time in milliseconds, or b) a Promise.

await after(100) // only a delay

await after(100, {
    updatedAt: '2020-04-01'
})
await after(100, () => ({
    updatedAt: new Date()
}))

after(credentialPromise, user => logIn(user))
    .then(token => /* */ )

after is an Observable!

after is also an Observable, so you can obtain its value via, subscribe() , and cancel its subscription by calling .unsubscribe() . A stream with any desired timing can be created by concat -enating a bunch of after s.

after(100, 3.14).subscribe(pi => (setDiameter(radius * 2 * pi)))

after(0, 3.14).subscribe(pi => Math.TAU = 2 * pi);
// Math.TAU is immediately available because its after '0'

concat(
    after(0, Math.PI),
    after(1000, 2.718)
).subscribe(constant)

Passing 0 msec causes after to be a synchronous Observable.

For e2e and unit testing, and Storybook, in order to simulate component interaction precisely, it is useful to create a script by concatenating after s of user interactions, or other function invocations.

const sub = concat(
    after(0, () => userEvent.type(email, "me@email.com")),
    after(1000, () => userEvent.type(password, "password123"))
).subscribe()

// unsubscribe, and unstarted steps will not run
sub.unsubscribe();

Unlike Promises, which run to completion by design (they are a Promise after all), an after or a concat-enation of afters is fully cancelable! So if you unsubscribe() on component unmount, for example, the chain will be halted wherever it is.

Also unlike Promises, the Observable model assumes nothing about whether the execution is sync or async, or whether yielding zero- single- or multiple -values. Notifications are simply delivered whenever the Observable calls observer.next().

after can delay an Observable!

When the valueProducer argument is an Observable, after returns a delayed subscription to the Observable.

In contrast, RxJS delay only delays the notifications. The difference is when the underlying resource is actually used at subscription time, or only after the delay.

Conclusion

That's all - may your successive explorations of after spark joy.