@smidhonza/either

A tiny TypeScript Either helper for values that can go one of two ways:

• Right(value) means the computation worked.
• Left(value) means it did not.

Think of it as a civilized alternative to throwing errors across the room.

Install

npm i @smidhonza/either

Simple Example

import { Either, Left, Right, isLeft } from '@smidhonza/either';

type ParseError = {
  message: string;
};

const parsePort = (input: string): Either<ParseError, number> => {
  const port = Number(input);

  if (!Number.isInteger(port)) {
    return Left({ message: `"${input}" is not a whole number` });
  }

  if (port < 1 || port > 65535) {
    return Left({ message: `${port} is not a valid port` });
  }

  return Right(port);
};

const result = parsePort('3000');

if (isLeft(result)) {
  console.error(result.value.message);
} else {
  console.log(`Server ready on port ${result.value}`);
}

Output:

Server ready on port 3000

Try parsePort('banana') and it will politely hand you a Left instead of turning your call stack into a crime scene.

API

Either<L, R>

A union type representing either a left value or a right value.

type Result = Either<Error, string>;

Left(value)

Creates a left value. Use it for errors, missing data, invalid input, or any branch where the answer is "not today".

const result = Left('User not found');

Right(value)

Creates a right value. Use it for successful results.

const result = Right({ id: 1, name: 'Ada' });

isLeft(either)

Checks whether an Either is a Left, while narrowing the TypeScript type.

if (isLeft(result)) {
  console.log(result.value);
}

isRight(either)

Checks whether an Either is a Right, while narrowing the TypeScript type.

if (isRight(result)) {
  console.log(result.value);
}

Development

Build the package:

npm run build

License

ISC