ts-res

Tiny library to improve error handling in TS using Rust-inspired types.

Contents

• Installation
• Result Type
• Helper Methods
• Examples

Installation

• Using npm - npm install ts-res
• Using yarn - yarn add ts-res
• Using pnpm - pnpm add ts-res

Result Type

Result type is convenient to use as a return type of a method that may fail.

In Result<T, E>:

• T specifies success result type, which can be any type
• E specifies error type, which is constrained by Error | string | void | undefined.

Err() and Ok() are used as a return type enforcers.

function mayFail(isFailed: boolean): Result<number, Error> {
  if (isFailed) {
    return Err(new Error("Function failed"));
  }

  return Ok(123);
}

As a result we get an object which will either be {ok: true, data: T} or {ok: false, error: E}

Note: Ok() and Err() can be used with no values if T/E were specified as void or undefined

function getEmptyResult(isFailed: boolean): Result<void, void> {
  if (isFailed) {
    return Err();
  }

  return Ok();
}

Helper Methods

Result<T, E> can be handled manually:

const result = mayFail(true);

if (!result.ok) {
  throw result.error;
}

console.log(result.data);

However it's much easier to use helper methods to quickly handle the result:

• throw(message?: string) - throws an error (with an optional custom error message) or returns an unwrapped result
• or(value: T) - returns an unwrapped result or a back-up value
• else(callback: (error: E) => T) - returns an unwrapped result or executes callback that returns back-up value which can be based on provided error
• and(callback: (result: T) => Result<T, E>) - handles a result in a callback while ignoring an error, returns the result allowing for chaining

// -- throw()
mayFail(true).throw(); // returns 123
mayFail(false).throw(); // throws an Error with default message
mayFail(false).throw("My Message"); // throws an Error with "My Message"

// -- or()
mayFail(true).or(100); // returns 123
mayFail(false).or(100); // returns 100

// -- else()
mayFail(true).else((error) => 200); // returns 123
mayFail(false).else((error) => 200); // returns 200 (error can be used for some extra logic)

// -- and()
mayFail(true).and((result) => { 
  console.log(result);
}); // logs 123
mayFail(false).and((result) => {
  console.log(result);
}); // doesn't do anything

Examples

More elaborate examples:

• Use throw() to unwrap the value after parsing a number

function toNumber(str: string): Result<number, string> {
  const result = Number(str);

  if (isNaN(result)) {
    return Err("Couldn't parse a string");
  }

  return Ok(result);
}

const myNumber: number = toNumber("Hello").throw(); // Throws an Error
const myNumber: number = toNumber("123").throw(); // Returns 123

• Use or() to provide a back-up value while obtaining status code

function getStatusCode(statusCode: number): Result<number, string> {
  if (statusCode > 200 && statusCode < 300) {
    return Ok(statusCode);
  }

  return Err("Invalid status code");
}

function obtainStatus(): number {
  return getStatusCode(response.statusCode).or(404); // Returns `statusCode` between 201 and 299 or 404
}

• Use else() and CustomError to provide a back-up value based on error type

enum ErrorType {
  A,
  B,
  C,
}

class CustomError extends Error {
  private errorType: ErrorType;

  constructor(type: ErrorType) {
    super();
    this.errorType = type;
  }

  get type(): ErrorType {
    return this.errorType;
  }
}

function myFunction(): Result<void, CustomError> {
  //...
}

myFunction().else((error) => {
  switch (error.type) {
    case ErrorType.A:
      return "a";
    case ErrorType.B:
      return "b";
    case ErrorType.C:
      return "c";
    default:
      break;
  }
});

• Use and() to handle a result of writing data to localStorage and ignore the error

function readLocalStorage(key: string): Result<string, void> {
  const data = localStorage.getItem(key);

  if (!data) {
    return Err();
  }

  return Ok(data);
}

readLocalStorage.and((data) => {
  const parsedData = JSON.parse(data);
  console.log(parsedData);
})

• Use and() for handling http response data and chaining it with another method

type User = {
  name: string;
  age: number;
};

async function fetchUserData(): Promise<Result<User, Error>> {
  const response = await fetch("https://api.example.com/user");

  if (!response.ok) {
    return Err(new Error("Failed to fetch user data"));
  }

  const data = await response.json();

  return Ok(data);
}

const result = await fetchUserData();

result
  .and((user) => {
     console.log(user);
  })
  .else((error) => {
    console.error(error);
  });