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Option, Result, and Either types for JavaScript

🦀 Inspired by Rust

Zero dependenciesLightweightFunctional


npm install @thames/monads

Getting started

The Option<T> type

Option represents an optional value: every Option is either Some and contains a value, or None, and does not.

[!NOTE] Full documentation here: Option

import { Option, Some, None } from '@thames/monads';

const divide = (numerator: number, denominator: number): Option<number> => {
if (denominator === 0) {
return None;
} else {
return Some(numerator / denominator);

// The return value of the function is an option
const result = divide(2.0, 3.0);

// Pattern match to retrieve the value
const message = result.match({
some: (res) => `Result: ${res}`,
none: 'Cannot divide by 0',

console.log(message); // "Result: 0.6666666666666666"

The Result<T, E> type

Result represents a value that is either a success (Ok) or a failure (Err).

[!NOTE] Full documentation here: Result

import { Result, Ok, Err } from '@thames/monads';

const getIndex = (values: string[], value: string): Result<number, string> => {
const index = values.indexOf(value);

switch (index) {
case -1:
return Err('Value not found');
return Ok(index);

const values = ['a', 'b', 'c'];

getIndex(values, 'b'); // Ok(1)
getIndex(values, 'z'); // Err("Value not found")

The Either<L, R> type

Either represents a value that is either Left or Right. It is a powerful way to handle operations that can result in two distinctly different types of outcomes.

[!NOTE] Full documentation here: Either

import { Either, Left, Right } from '@thames/monads';

const divide = (numerator: number, denominator: number): Either<string, number> => {
if (denominator === 0) {
return Left('Cannot divide by 0');
} else {
return Right(numerator / denominator);

const result = divide(2.0, 3.0);

const message = result.match({
left: (err) => `Error: ${err}`,
right: (res) => `Result: ${res}`,

console.log(message); // "Result: 0.6666666666666666"