TypeScript Optional

Optional (like Java) implementation in TypeScript

Overview

Optional<T> is a type which may or may not contain a payload of type T. It provides a common interface regardless of whether an instance is present or is empty.

This module is inspired by Optional class in Java 8+.

The following methods are currently not supported:

• equals
• toString
• hashCode
• stream

Install

npm install --save typescript-optional

Usage

import

// import Optional type from this module
import { Optional } from "typescript-optional";

creating Optional<T> objects

const nullableString: string | null = /* some nullable value */;

// all the following variables will be parameterized as Optional<string>.
const optional = Optional.ofNullable(nullableString);
const optionalPresent1 = Optional.ofNullable("foo");
const optionalPresent2 = Optional.ofNonNull("foo"); // accepts non-null value (or else throws TypeError)
const optionalEmpty1: Optional<string> = Optional.empty(); // type hinting required
const optionalEmpty2 = Optional.empty<string>(); // or parameterize explicitly

operations

const optional: Optional<string> = Optional.ofNullable( /* some optional value: null | string */ );

// force to retrieve the payload. (or else throws TypeError.)
// this method is not used match.
optional.get();

// represent whether this is present or not.
optional.isPresent();

// represent whether this is empty or not. (negation of `isPresent` property)
optional.isEmpty();

// if a payload is present, execute the given `consumer`.
optional.ifPresent(value => console.log(value));

// if a payload is present, execute the first argument (`consumer`),
// otherwise execute the second argument (emptyAction).
optional.ifPresentOrElse(value => console.log(value), () => console.log("empty"));

// filter a payload with additional predicate.
optional.filter(value => value.length > 0);

// map a payload with the given mapper.
optional.map(value => value.length);

// map a payload with the given mapper which returns value wrapped with Optional type.
const powerIfPositive: (x: Number) => Optional<Number>
    = x => (x > 0) ? Optional.ofNonNull(x * x) : Optional.empty();
const numberOptional: Optional<null | number> = Optional.ofNullable(/* some optional value: null | number */)
numberOptional.flatMap(value => powerIfPositive(value as number));

// if this is present, return this, otherwise return the given another optional.
const another: Optional<string> = Optional.ofNullable(/* ... */);
optional.or(another);

// if this is present retrieve the payload,
// otherwise return the given value.
optional.orElse("bar");

// if a payload is present, retrieve the payload, 
// otherwise return a value supplied by the given function.
optional.orElseGet(() => "bar");

// if a payload is present, retrieve the payload,
// otherwise throw an exception supplied by the given function.
optional.orElseThrow(() => new Error());

// if a payload is present, retrieve the payload,
// otherwise return null.
optional.orNull();

// if a payload is present, retrieve the payload,
// otherwise return undefined.
optional.orUndefined();

// return an appropriate result by emulating pattern matching with the given cases.
optional.matches({
    present: value => value.length,
    empty: () => 0, 
})

// convert this to an Option value.
optional.toOption();

prototype-free types

While Optional's fluent interface for method chaining with prototype is usually useful and elegant, relying on prototype can cause some problems in certain situations like that an external function copies such objects except prototype. For example, setState of React reflects the given value as a state except the value's prototype (and then you will see "TypeError: undefined is not a function" in runtime though TypeScript compilation has been succeeded!).

To avoid this issue, you have three options that convert an Optional into a prototype-free, or a simple JavaScript object (associative array, string etc.).

• Optional.orNull
• Optional.orUndefined
• Optional.toOption

Optional.orNull and Optional.orUndefined

Using Optional.orNull or Optional.orUndefined is the simple way to obtain prototype-free objects. These methods convert an Optional<T> into a value of type union.

Optional<T>.orNull returns T | null.

Optional<T>.orUndefined returns T | undefined. The T | undefined type is compatible with optional parameters and properties of TypeScript.

Use Optional.ofNullable to restore an Optional value from a value of these type unions.

const update: <T> (original: T) => T = /* some external function that returns without the prototype */
const optional: Optional<string> = /* some Optional object */;

let nullable: string | null = optional.orNull();
let orUndefined: string | undefined = optional.orUndefined();

// update using external functions!
nullable = update(nullable);
orUndefined = update(orUndefined);

// retore from (string | null).
const optionalFromNullable: Optional<string> = Optional.ofNullble(nullable);

// restore from (string | undefined).
const optionalFromOrUndefined: Optional<string> = Optional.ofNullble(orUndefined);

Option.toOption

As a more explicit way to obtain prototype-free objects, Optional.toOption is provided. This method convert an Optional<T> into an object of Option<T> type, which conforms to discriminated unions also known as algebraic data types. Refer the API document of Option<T> to learn about the structure.

const update: <T> (original: Option<T>) => T = /* some external function that returns without the prototype */
const optional: Optional<string> = /* some Optional value */;

let option: Option<string> = optional.toOption();

// update using external functions!
option = update(option);

// restore from Option<T>.
const optionalFromOption: Optional<string> = Optional.from(option);

License

MIT License - LICENSE.md