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https://github.com/api3dao/promise-utils

A simple package for a functional and typesafe error handling
https://github.com/api3dao/promise-utils

promise typescript utilities

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A simple package for a functional and typesafe error handling

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# promise-utils [![ContinuousBuild](https://github.com/api3dao/promise-utils/actions/workflows/main.yml/badge.svg?branch=main)](https://github.com/api3dao/promise-utils/actions/workflows/main.yml)



> A simple package for a functional and typesafe error handling with zero dependencies



## Installation



To install this package run either:



`yarn add @api3/promise-utils`



or if you use npm:



`npm install @api3/promise-utils --save`



## Usage



The API is small and well focused on providing [more concise error handling](#motivation). The main functions of this

package are `go` and `goSync` functions. They accept a function to execute, and additionally `go` accepts an optional

`GoAsyncOptions` object as the second parameter. If the function executes without an error, a success response with the

data is returned, otherwise an error response is returned.



```ts

// Imagine an async function for fetching API data

const goFetchData = await go(() => fetchData('users'));

// The "goFetchData" value is either: {success: true, data: ...} or {success: false, error: ...}

if (goFetchData.success) {

  const data = goFetchData.data

  ...

}

```



or:



```ts

// Imagine an async function for fetching API data

// If the fetch data is a non class function returning a promise, you can drop the arrow function

const goFetchData = await go(() => fetchData('users'));

// The "goFetchData" value is either: {success: true, data: ...} or {success: false, error: ...}

if (!goFetchData.success) {

  const error = goFetchData.error

  ...

}

```



and with `GoAsyncOptions`:



```ts

// The `fetchData` function will be retried a maximum of 2 times on error, with each attempt having

// a timeout of 5 seconds and a total timeout 10 seconds (shared among all attempts and delays).

const goFetchData = await go(() => fetchData('users'), { retries: 2, attemptTimeoutMs: 5_000, totalTimeoutMs: 10_000 });

...

```



and for synchronous functions:



```ts

const someData = ...

// Imagine a synchronous function for parsing data

const goParseData = goSync(() => parseData(someData));

// The goParseData value is either: {success: true, data: ...} or {success: false, error: ...}

if (goParseData.success) {

  const data = goParseData.data

  ...

}

```



The return value from the promise utils functions works very well with TypeScript inference. When you check the the

`success` property, TypeScript will infer the correct response type.



## API



The full `promise-utils` API consists of the following functions:



- `go(asyncFn, options)` - Executes the `asyncFn` and returns a response of type `GoResult`

- `goSync(fn)` - Executes the `fn` and returns a response of type `GoResult`

- `assertGoSuccess(goRes)` - Verifies that the `goRes` is a success response (`GoResultSuccess` type) and throws

  otherwise.

- `assertGoError(goRes)` - Verifies that the `goRes` is an error response (`GoResultError` type) and throws otherwise.

- `success(value)` - Creates a successful result value, specifically `{success: true, data: value}`

- `fail(error)` - Creates an error result, specifically `{success: false, error: error}`



and the following Typescript types:



- ```ts

  type GoResult = { data: T; success: true };

  ```

- ```ts

  type GoResultSuccess = { error: E; success: false };

  ```

- ```ts

  type GoResultError = GoResultSuccess | GoResultError;

  ```

- ```ts

  interface GoAsyncOptions {

    retries?: number; // Number of retries to attempt if the go callback is unsuccessful.

    attemptTimeoutMs?: number | number[]; // The timeout for each attempt. Can provide an array for different timeouts for each attempt. If the array is shorter than the number of retries, the last value is used for all remaining attempts, if the length of the array is longer than the number of retries, the extra values are ignored.

    totalTimeoutMs?: number; // The maximum timeout for all attempts and delays. No more retries are performed after this timeout.

    delay?: StaticDelayOptions | RandomDelayOptions; // Type of the delay before each attempt. There is no delay before the first request.

    onAttemptError?: (goRes: GoResultError) => void; // Callback invoked after each failed attempt is completed. This callback does not fire for the last attempt or when a "totalTimeoutMs" is exceeded (these should be handled explicitly with the result of "go" call).

  }

  ```

- ```ts

  interface StaticDelayOptions {

    type: 'static';

    delayMs: number;

  }

  ```

- ```ts

  interface RandomDelayOptions {

    type: 'random';

    minDelayMs: number;

    maxDelayMs: number;

  }

  ```



Careful, the `attemptTimeoutMs` value of `0` means timeout of 0 ms. If you want to have infinite timeout omit the key or

set it to `undefined`.



The last exported value is a `GoWrappedError` class which wraps an error which happens in go callback. The difference

between `GoWrappedError` and regular `Error` class is that you can access `GoWrappedError.reason` to get the original

value which was thrown by the function.



Take a look at the [implementation](https://github.com/api3dao/promise-utils/blob/main/src/index.ts) and

[tests](https://github.com/api3dao/promise-utils/blob/main/src/index.test.ts) for detailed examples and usage.



## Motivation



### Verbosity and interoperability of try-catch pattern



```ts

// Verbose try catch

try {

  const data = await someAsyncCall();

  ...

} catch (e) {

  // The "e" is "unknown" because any value can be thrown in Javascript so casting is needed

  return logError((e as MyError).reason);

}



// Compare it to simpler version using go

const goRes = await go(someAsyncCall);

if (!goRes.success) return logError(goRes.error.reason);

// At this point TypeScript infers that the error was handled and "goRes" must be a success response

const data = goRes.data;

...

```



Also, think about what happens when you want to handle multiple "can fail" operations in a single function call. You can

either:



1. Have them in a same try catch block - but then it's difficult to differentiate between what error has been thrown.

   Also this usually leads to a lot of code inside a try block and the catch clause acts more like "catch anything".

2. Use nested try catch blocks - but this hurts readability and forces you into the

   [callback hell pattern](http://callbackhell.com/).



### Consistent throwing of an `Error` instance



JavaScript supports throwing any expression, not just `Error` instances. This is also a reason why TypeScript infers the

error as `unknown` or `any` (see:

[useUnknownInCatchVariables](https://www.typescriptlang.org/tsconfig#useUnknownInCatchVariables)).



The error response from `go` and `goSync` always return an instance of the `Error` class. Of course, throwing custom

errors (derived from `Error`) is supported.



### Intentionally limited feature set



The go utils by design offer only very basic timeout and retry capabilities as these are often application specific and

could quickly result in bloated configuration. If you are looking for more complex features, consider using one of the

alternatives, e.g. https://github.com/lifeomic/attempt



## Limitations



There is a limitation when using class functions due to how javascript

[this](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this) works.



```ts

class MyClass {

  constructor() {}

  get() {

    return this._get();

  }

  _get() {

    return '123';

  }

}



const myClass = new MyClass();

const resWorks = goSync(() => myClass.get()); // This works

// However, seeing the line above it may be tempting to rewrite it to

const resFails = goSync(myClass.get); // This doesn't work

```



The problem is that the `this` keyword is determined by how a function is called and in the second example, the `this`

inside the `get` function is `undefined` which makes the `this._get()` throw an error.



## Developer documentation



### Release



To release a new version follow these steps:



1. `yarn && yarn build`

2. `yarn version` and choose the version to be released

3. `yarn publish --access public`

4. `git push --follow-tags`