Resolv.js

a small promise handler.

Abstract

This is a small utility to make handling promises a little more readable. Avoiding Towers of Terror and Pyramids of Doom.

Usage:

resolve() takes the promise to resolve as its first argument, optionally takes a boolean for the second parameter; convertToError, finally it optionally takes the parameters of JSON.stringify (save for the first) as the remaining parameters, to be used when stringifying errors.

If convertToError is

• true, and an error is given that's not an instance of Error, resolve() will return an instance of Error with the given error stringified on the message property.
• given a falsy value, instances of Object and Error will be returned as-is, while primitives will be treated as if true was provided. This is the default behavior.
• explicitly false, the error will always be returned as-is. If you wish for this to be the default behavior, import the named export resolve rather than using the default export.

resolve() returns a tuple; [res, err]. Where res is the result of the promise, and err is an error, if one was encountered. The values are mutually exclusive, so if one is defined, the other will be undefined.

JavaScript:

import resolve from "resolvjs"
import anAsyncFunction from "some-api-client"

const [res, err] = await resolve(anAsyncFunction())
if (err) throw err // or handle it in some other way

console.log(res)

TypeScript:

resolve() is a generic function, enabling you to specify the type of the result in such cases where it cannot be or is incorrectly inferred. However, the returned type for res will always be R | undefined, this allows you to use a guard pattern, as shown below, to ensure that errors are always handled. After which, res will be of type R.

import resolve from "resolvjs"
import anAsyncFunction from "some-api-client"
import type { SuccessResponse } from "some-api-client/types"

const [res, err] = await resolve<SuccessResponse>(anAsyncFunction())
if (err) throw err // or handle it in some other way

console.log(res)

or:

resolve() can also take a type parameter for err. When using the default export, this results in err being of type E | Error | undefined, the example below shows how err can be inferred as E. When using the named export and no type parameter is provided, err will be of type unknown. However if a type parameter is provided, it will be of type E | undefined.

import resolve from "resolvjs"
import anAsyncFunction from "some-api-client"
import type { SuccessResponse, ErrorResponse } from "some-api-client/types"

const [res, err] = await resolve<SuccessResponse, ErrorResponse>(anAsyncFunction(), false)
if (err) {
    if ("errorProp" in err) {
        throw err.errorProp
    }
    throw err
}

console.log(res)

Inspiration

Inspiration for this project came from the Fireship short Async Await try-catch hell and uses the solution presented nearly verbatim.