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A Typescript implementation of the Maybe data type
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Maybe encapsulates the idea of a value that might not be there.

A Maybe value can either be Just some value or Nothing.

type Maybe<T> =
  | Just<T>
  | Nothing

Where Nothing is an instance of Nothing, null or undefined, and Just represents any non Nothing value.


npm install --save


import { Maybe, just, withDefault, map2 } from '';

interface User {
  email: string;
  name: Maybe<string>;
  surname: Maybe<string>;
const user: User = {
  email: '',
  name: just('John'),
  surname: just('Doe')

const getFullName = (name: string, surname: string) => `${name} ${surname}`;
const maybeFullname = map2(getFullName,, user.surname); // Just<string>('John Doe')
const fullName = withDefault(maybeFullname, ''); // 'John Doe'


(Inspired by elm-lang)


just<T>(value: T): Just<T>

Wraps a value in an instance of Just.

just(5); // Just<number>(5)


nothing(): Nothing

Creates an instance of Nothing.

nothing(); // Nothing


isJust(value: Maybe<any>): boolean

Returns true if a value is an instance of Just.

isJust(nothing()); // false


isNothing(value: Maybe<any>): boolean

Returns true if a value is an instance of Nothing.

isNothing(just(5)); // false
isNothing(undefined) // true
isNothing(null) // true
isNothing(nothing()) // true


withDefault<A>(value: Maybe<A>, defaultValue: A): A

If value is an instance of Just it returns its wrapped value, if it's an instance of Nothing it returns the defaultValue.

withDefault(just(5), 0); // 5
withDefault(nothing(), 'hola'); // 'hola'


caseOf = <A, B>(caseof: { Just: (v: A) => B; Nothing: () => B; }, value: Maybe<A>): B

Run different computations depending on whether a Maybe is Just or Nothing.

  Nothing: () => 'zzz',
  Just: n => `Launch ${n} missiles`
}, just(5)); // 'Launch 5 missiles'


map<A, B>(f: (a: A) => B, value: Maybe<A>): Maybe<B>

Transforms a Maybe value with a given function.

const add1 = (n: number) => n + 1;
map(add1, just(4)); // Just<number>(5)
map(add1, nothing()); // Nothing

There are map2, map3, map4, map5 and mapN if you need to parametrize the function with more than one argument. (mapN only accepts arguments of the same type).

map2<A, B, C>(
  f: (a: A, b: B) => C, 
  a: Maybe<A>, b: Maybe<B>
): Maybe<C>
map3<A, B, C, D>(
  f: (a: A, b: B, c: C) => D, 
  a: Maybe<A>, b: Maybe<B>, c: Maybe<C>
): Maybe<D>
map4<A, B, C, D, E>(
  f: (a: A, b: B, c: C, d: D) => E, 
  a: Maybe<A>, b: Maybe<B>, c: Maybe<C>, d: Maybe<D>
): Maybe<E>
map5<A, B, C, D, E, F>(
  f: (a: A, b: B, c: C, d: D, e: E) => F, 
  a: Maybe<A>, b: Maybe<B>, c: Maybe<C>, d: Maybe<D>, e: Maybe<E>
): Maybe<F>
mapN<A, B>(
  f: (...a: A[]) => B, 
  ...a: Maybe<A>[]
): Maybe<B>


andThen = <A, B>(f: (a: A) => Maybe<B>, v: Maybe<A>): Maybe<B>

Chains together many computations that may fail.

const head = (arr: string[]) => arr.length > 0 ? just(arr[0]) : nothing();
andThen(head, just(['a','b','c'])); // Just<string>('a')
andThen(head, just([])); // Nothing
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