ts-error-as-value

Typescript error as value

---

Install

yarn install ts-error-as-value

---

(Optionally) Make functions and types global

import "ts-error-as-value/lib/globals";

This will make the functions ok and err, as well as the types Ok, Err and Result globally available

---

Result type

type None = null;

type Err<T, E extends Error = Error> = {
  data: never,
  error: E,
  get errorStack(): Error[],
  unwrap(): void, // Returns the value, but throws an error if the result is an Error
  unwrapOr<D>(defaultValue: D): D, // Returns the value or gives you a default value if it's an error
  mapErr<E2 extends Error>(fn: (err: E) => E2): Err<T, E2>, // If the result is an error, map the error to another error
  andThen<N>(fn: (data: never) => N): Err<T, E> // If the result is not an error, map the data in it
};

type Ok<T> = {
  data: T,
  error: never,
  get errorStack(): never,
  unwrap(): T, // Returns the value, but throws an error if the result is an Error
  unwrapOr<D>(defaultValue: D): T, // Returns the value or gives you a default value if it's an error
  mapErr<E2 extends Error>(fn: (err: never) => E2): Ok<T>, // If the result is an error, map the error to another error
  andThen<N>(fn: (data: T) => N): Ok<N> // If the result is not an error, map the data in it
};

type Result<
  T, E extends Error = ErrorResult
> = Err<T, E> | Ok<T>;

---

Functions

const err: <E extends Error>(error: E) => Err<E>;
const ok: <T>(data: T) => Ok<T>;

---

Basic Usage

Wrap the returns from functions with err for errors, and ok for non-error returns so that the function calling it receives a Result type.

const fnWithResult = (): Result<string, Error> => {
  if ("" !== "") {
    return ok("hello");
  }
  return err(new Error("Method failed"));
};

const { data, error } = fnWithResult();

if (error) {
  // is an error
} else {
  // guaranteed to not be an error, and typescript knows this
}

---

Or with promises:

const fnWithResult = async (): Promise<Result<string, Error>> => {
  if ("" !== "") {
    return ok("hello");
  }
  return err(new Error("Method failed"));
};

const callsFnThatCallsFnWithResult = async () => {
  const { data, error, errorStack } = (await fnWithResult())
  if (error) {
    return err(error);
  }
  return ok(data);
};

callsFnThatCallsFnWithResult();

---

Methods on Result

class NewError extends Error {}

const fnWithResult = (): Result<string, Error> => {
  if ("" !== "") {
    return ok("hello");
  }
  return err(new Error("Method failed"));
};

const { data, error } = fnWithResult()
    .mapErr(error => new NewError("Failed to call fnWithResult"))
    .andThen(data => {
      return data === "hello";
    });

Because of using andThen, data will be boolean if fnWithResult returns a value.

Because of mapErr, error will be an instance of NewError if fnWithResult returns an error