TypeScript Nullable

What is This?

Glad you asked. This is a type-safe formalization of the concept of possibly absent values in TypeScript. It is perhaps even more importantly a module of type-safe utility functions that deal with possibly absent values.

Think of it roughly like a JavaScript-friendly version of Haskell’s or Elm’s Maybe type and corresponding module of functions for dealing with Maybe. It is functional to its core, with typed and curried pure functions.

Installation

From the command line:

$ npm install --save typescript-nullable

In your TypeScript files:

import { Nullable } from 'typescript-nullable'

Nullable Type Definition

type None = null | undefined

type Nullable<T> = T | None

Module Utility Functions

This module also ships with a Nullable object that contains multiple useful functions for dealing with potentially absent values. Thus we have both a Nullable type and a Nullable object of utility functions.

All utility functions are curried to the extent that their final argument is optional. If a final argument is not provided, the function will return another function that expects that final argument.

Nullable.isNone

Determines if a provided Nullable is None and provides a type guard.

Type Annotation

<T>(nullable: Nullable<T>): nullable is None

Example Usage

Nullable.isNone('noob noob') // false
Nullable.isNone(null) // true
Nullable.isNone(undefined) // true

const possiblyNullValue: Nullable<string> = 'noob noob'

if (Nullable.isNone(possiblyNullValue)) {
  // in this scope, TypeScript knows possiblyNullValue is a None
}

Nullable.isSome

Determines if a provided Nullable is a concrete value and provides a type guard.

Type Annotation

<T>(nullable: Nullable<T>): nullable is T

Example Usage

Nullable.isNone('noob noob') // true
Nullable.isNone(null) // false
Nullable.isNone(undefined) // false

const possiblyNullValue: Nullable<string> = 'noob noob'

if (Nullable.isSome(possiblyNullValue)) {
  // in this scope, TypeScript knows possiblyNullValue is a concrete string
}

Nullable.map

Applies the provided function to the provided Nullable only if it is not None. Returns null otherwise.

Type Annotation

<A, B>(func: (val: A) => B): (nullable: Nullable<A>) => Nullable<B>

Example Usage

const toUpper = (text: string): string => text.toUpperCase()
Nullable.map(toUpper, 'noob noob') // NOOB NOOB
Nullable.map(toUpper, null) // null
Nullable.map(toUpper, undefined) // null

Nullable.withDefault

Provided a default value and a Nullable, will return the default value when the Nullable is None. Will return the concrete value of the Nullable if it is, in fact, concrete.

Type Annotation

<T>(defaultVal: T): (nullable: Nullable<T>) => T

Example Usage

Nullable.withDefault('morty')('rick') // 'rick'
Nullable.withDefault('morty')(null) // 'morty'

Nullable.maybe

Provided a default value, a function, and a Nullable, will return the default value when the Nullable is None. Will return the provided function applied to the concrete value of the Nullable if it is, in fact, concrete.

Type Annotation

<A, B>(defaultVal: B, f: (a: A) => B): (nullable: Nullable<A>) => B

Example Usage

import { add } from 'ramda'
Nullable.maybe(7, add(83), null)) // 7
Nullable.maybe(7, add(83), 34)) // 117

Nullable.andThen

Used for chaining functions that take a raw value of type T but return a Nullable<T>. This is like Haskell's bind or >>=.

Type Annotation

Nullable.andThen<A, B>(func: (val: A) => Nullable<B>): (nullable: Nullable<A>) => Nullable<B>

Example Usage

import { compose, curry } from 'ramda'

// Some arbitrary function that returns a Nullable:
const safeDivide = curry((a: number, b: number): Nullable<number> => {
  return a === 0
    ? null
    : b / a
})

compose(
  Nullable.andThen(safeDivide(3)),
  Nullable.andThen(safeDivide(0)), // this line results in a None value so the rest of the composition chain passes along None without blowing up or throwing an exception
  Nullable.andThen(safeDivide(4)),
  safeDivide(2),
)(32) // null

compose(
  Nullable.andThen(safeDivide(3)),
  Nullable.andThen(safeDivide(5)),
  Nullable.andThen(safeDivide(4)),
  safeDivide(2),
)(32) // 0.5333333333333333

Nullable.ap

Used for writing in the applicative style. For "lifting" functions into the Nullable context.

Type Annotation

Nullable.ap<A, B>(targetNullable: Nullable<A>): (applicativeNullable: Nullable<(val: A) => B>) => Nullable<B>

Example Usage

// Some arbitrary curried function that takes 3 concrete values:
const addThreeNumbers = (a: number) => (b: number) => (c: number) => a + b + c

compose(
  Nullable.ap(3),
  Nullable.ap(2),
  Nullable.ap(1),
)(addThreeNumbers) // 6

// This can be thought of as "lifting" addThreeNumbers into the context of its passed-in arguments being Nullable:
compose(
  Nullable.ap(3),
  Nullable.ap(null as Nullable<number>), // note we have to typecast this here because TypeScript can’t be sure what kind of Nullable<T> it has at this point.
  Nullable.ap(1),
)(addThreeNumbers) // null