Simple Error Either in TypeScript

When writing a function, I often want to return either the successful value or an error. Given that my current project is in TypeScript, we came up with a nice little Either pattern for returning successes or errors.

Since writing this pattern, I’ve found myself reaching for it often. We define a Result to be either T or an Error. By using a simple isError helper, we can prove our result to TypeScript and maintain type safety.

type Result<T> = T | Error;
export type Type<T> = Result<T>;

export function isError<T>(result: Result<T>): result is Error {
  return result instanceof Error;
}

export function isSuccess<T>(result: Result<T>): result is T {
  return !isError(result);
}

This approach is especially nice since it is type-safe and doesn’t require any wrapping object. It’s a simple snippet, but very useful; try it out on your next TypeScript project! One caveat: this pattern will not work with an any type (it will report all things as success).

Usage

  import * as Result from 'result';
  type Thing = {
    name: string
  }
  function doIt(): Result.Type<Thing>{
    if(true) {
      return {name: "Todd"};
    } else {
      return new Error("boom")
    }
  }
  const errorOrSuccess = doIt();
  if(isError(errorOrSuccess)) {
    // now we can know that we have an error
    console.log(errorOrSuccess.message)
  } else {
    console.log(errorOrSuccess.name)
  }