outputs-not-outcomes

Using Javascript Promises as Results

Install

npm i -S outputs-not-outcomes

Usage

This package is to be used exactly like Promise, the only difference being the typing of errors.

Constructing a type-safe result

const result = new Result<MyCustomError, MyResult>((success, failure) => {
  success('my success here')
  // or
  failure(new MyCustomError())
})

const resolved = Result.resolve(1) // Result<never, number>

const rejected = Result.reject(1) // Result<number, never>

Building pipelines

See register.ts, roughly:

function register(email: string, password: string) {
  return validatePassword(password) // sync check
    .then(() => validateEmail(email))
    .then(() => assertNoAccountExistsWithEmail(email)) // async check
    .then(() => createAccount(email, password)) // async task
}

FAQ

Why do I sometimes have RuntimeError and sometimes not ?

It depends on how you constructed the result. For instance, Result.resolve(1) is entirely safe, there's no chance a type error or whatever would slip in the 1 statement.

However when resolving a Result from a Promise (or PromiseLike), we have no guarantee that the underlying Promise or PromiseLike does not embed any runtime error. Since there's a risk, there's a possibility. Therefore is it is typed.

What can introduce a runtime error:

• new Result((resolve, reject) => { … }) -> the function body may contain a runtime error.
• Result.resolve(promiseLike) -> promiseLike may contain a runtime error.
• Result.reject(promiseLike) -> promiseLike may contain a runtime error.
• myResult.then(a, b) -> a or b function bodies may contain a runtime error.
• myResult.catch(cb) -> cb function body may contain a runtime error.