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It's Like a Promise with typed errors and other improvements that leads to more robust code.

made (and best used) with TypeScript ♥️


Install the library in your project.

npm install @ts-task/task

Use it in your code preeeeety much how you would use a Promise.

import { Task } from '@ts-task/task';

// Create it with a resolver
const task1 = new Task((resolve, reject) => {
        _ => resolve('Hello'),

// Or with a constructor
const task2 = Task.resolve('world');

Task.all([task1, task2])
    // Transform the eventual value
    .map(([msg1, msg2]) => `${msg1} ${msg2}!!!`)
    // And then do something with it
        err => console.error('Buu', err), // Errors come first!
        msg => console.log('Yeay', msg)


Promises are great! so why do we need a replacement? or when should I use Task?

Good question, I'm happy you asked:

  • Task have better error handling, so you'll have less bugs
  • Task are pipeable, so they are easier to extend
  • Task has more specific semantics, so it will be easier to know what you are doing
  • Task are Lazy, so it's easier to create retry logic

Better Error Handling

If you ever used Promises in TypeScript you may have noticed that it's only typed on success a.k.a Promise<T>. For example in the following code

const somePromise: Promise<string>;

  // Transform the eventual value
  .then(x => `${x}!!!`)
  // And then do something with it
    value => /* value is of type string */,
    err   => /* err is of type any */

We know value is of the expected type T but we don't know anything about err. The main reason is that when we transform our promises, the callbacks we pass to then can throw anything, and in TypeScript the exceptions are not typed. We could manually define the error type and say it's a Promise<string, Error> but it would be a lie because we can't avoid exceptions and if they happen we can't know their types, and because any & Error = any we can't be more specific.

Thats a boomer because we make all this trouble with static typings to have more confidence on how we program and we are left wide open when things go south.

So we can't forbid a function from throwing but we can wrap exceptions inside an UnknownError object, and with that decision alone we can type Task<T, E> and let TypeScript help us infer and manipulate errors 🎉.

For example

const task1 = Task.resolve(1);
// task1 is of type Task<number, never>, which makes sense as there is no way resolve can fail

const task2 = => '' + n)
// task2 is of type Task<string, UnknownError>, because we have converted the success value
// and we don't know if the inner function throws an error or not

You can also add, remove and transform your error logic and the type inference will get you a long way.

For example if you use the caseError function from @ts-task/utils you could do something like this

import { Task } from 'ts-task/task';
import { caseError, isInstanceOf } from 'ts-task/utils';

// Assuming we have defined a getUser function somewhere
declare function getUser(id: number): Task<User, DbError | UserNotFound>;

const user = getUser(100)
      // If the error meets this condition
      // Handle it with this callback
      err => Task.resolve(new Guest())
// user will have type Task<User | Guest, DbError | UnknownError> because caseError only handles
// UserNotFound (removing it from the errors) and resolves it to a new type of answer (Guest)
// and there is always the possibility that one of those functions throws, so we have to take UnknownError
// into account

Another use case could be to only retry an http request if the error was 502, or 504 which may happen on a timely basis but don't retry if the error was 401 or a rate limit as the expected result is the same.

When you fork a Task, the error callback comes first, so whenever you want to use the eventual value, you first need to decide what you do with the error. You can always ignore it or console.log it, but you need to make a conscious decision.

      err => console.error('Buu', err), // Errors come first!
      msg => console.log('Yeay', msg)

Pipe operator

When the pipe operator lands to JavaScript (currently in stage 1) we will be able to write code like this

const task = Task.resolve(1)
    |> map(n => '' + n)
    |> chain(getUser)
    |> retryWhen(DbError)
    |> catchError(caseError(UserNotFound, err => Task.resolve(null)))

which has the advantage of using custom methods without having to modify the prototype of Task. But because we cannot wait until the operator makes it to the standard we added a pipe method inspired by RxJs pipeable operators.

So the previous code would look like:

const task = Task.resolve(1).pipe(
      map(n => '' + n)
    , chain(getUser)
    , retryWhen(DbError)
    , catchError(caseError(UserNotFound, err => Task.resolve(null)))

which is not that different. All that's required is that the functions passed pipe to have the signature Task<T1, E1> => Task<T2, E2>. You can find a common operators in the @ts-task/utils library, but we encourage you to write your own.

Specific semantics

Promises API is quite simple by design, it has a then method that can be used for 3 different purposes, in contrast Task has a different method for each usage.

  • Promise.then can be used to transform an eventual value, with task you should use map.
  • Promise.then can be used to chain sequential async operations, with task you should use chain.
  • Promise.then can be used to do something once you have the result, with task you should use fork.

As stated in Lord of the Promises

One method to rule them all, One method to find them, One method to bring them all and in the darkness bind them

The nice thing about having one method should be simplicity (less methods to remember), but trying to put the different use cases in the same method can cause some confusions that we'll explain in this section.

When we want to do something with an eventual value, we need to know if the Promise succeeds or fails. Thats why then accepts two arguments, the onSuccess and onError callbacks (in that order).

If it's used in the middle of a Promise chain, it can cause some confusion

// It's not recommended to do this
  .then(x => foo(x))
      y   => bar(y),
      err => handleError(err)
  .then(z => baz(z))

a common doubt arises with handleError, does it catch errors on foo or in bar?. The answer is the first option, thats why it's recommended to write an explicit catch instead.

// Instead do this
  .then(x => foo(x))
  .catch(err => handleError(err))
  .then(y => bar(y))
  .then(z => baz(z))

But just the fact that you can write the previous code can be misleading.

The second argument of then should only be used in the last step of a Promise chain, when we are doing something with the result.

    html => render(html),
    err => openErrorModal(err)

But because the second parameter is optional, it's fairly easy to end up with fragile code. For example:

    html => render(html)

if somePromise fails there is no handler, depending on the environment you could get a silent error or an Uncaught Promise Rejection that may be difficult to trace.

When using task, if you want to do something with your eventual result you have to use fork as the last step. That method is the only one that doesn't return a new task, so it's impossible to use it in the middle of the chain. Even more, fork handles errors in the first callback, so it's impossible to have an Uncaught Promise Rejection.

      err => openErrorModal(err),
      html => render(html)

And because Tasks are lazy, if you don't call fork nothing happens, so the library forces you to use best practices.

The difference between map and chain is a little more subtle. Promise (and Tasks) transformations are useful when you don't care about the eventual value itself, rather something that can be synchronously computed from that value. For example, you could fetch a document and only care about how many words the document has.

  .then(doc => countWords(doc))
  .then(n => alert(`The document has ${n} words`));

  .map(doc => countWords(doc))
    n => alert(`The document has ${n} words`)

where we assume countWords is a function that receives a String and returns a Number.

In contrast, chaining Promises and Tasks are useful when you need the result of a previous async operation in order to make the next one. For example the first request may return a JSON object with an url to make the next request.

  .then(obj => analyzeResponseAndGetTheUrl(obj))
  .then(url => fetch(url));
  .then(obj => alert(`Some data ${}`));

  .map(obj => analyzeResponseAndGetTheUrl(obj))
  .chain(url => fetchTask(url));
    obj => alert(`Some data ${}`)

In this example we are synchronously transforming the first response using the analyzeResponseAndGetTheUrl function that receives an Object and returns a String and then chaining the eventual transformation with the second call to fetch.

This should give you an intuition that whenever you see a map you are not adding more time to your computation, but when you are using chain you are most likely are.


Initialized with @alexjoverm's TypeScript Library Starter under the Acamica Labs initiative and made with ❤️ by

Hernan Rajchert

💻 🎨 📖 💡 ⚠️

Gonzalo Gluzman

💻 🤔 ⚠️

This project follows the all-contributors specification. Contributions of any kind welcome!


If you only type on success, you are missing half the fun




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