Option<Value>

The Option type can be used as a replacement for null and undefined when manipulating optional data. Contrary to null and undefined, an option is kind of like a box, that contains a value or not.

It can be useful to distinguish values between each other: you can represent Some(None) with options, whereas undefined or null replace the value they intend to make optional.

An option can have two possible states:

• Some(value)
• None

Create an Option value

To create an option, use the Some and None constructors:

import { Option } from "@swan-io/boxed";

const aName = Option.Some("John");
const bName = Option.None();

// You can enforce the type using a type parameter
Option.Some<string>("John");
Option.None<string>();

You get interop with null and undefined:

// `value` being `null` or `undefined` makes a `None`
const a = Option.fromNullable(value);

// `value` being `null` makes a `None`
const b = Option.fromNull(value);

// `value` being `undefined` makes a `None`
const c = Option.fromUndefined(value);

Methods

The option type provides a few manipulation functions:

.map(f)

Option<A>.map<B>(f: (value: A) => B): Option<B>

If the option is Some(value) returns Some(f(value)), otherwise returns None.

Examples

Option.Some(2).map((x) => x * 2);
// Option.Some<4>

Option.None().map((x) => x * 2);
// Option.None

.flatMap(f)

Option<A>.flatMap<B>(f: (value: A) => Option<B>): Option<B>

If the option is Some(value) returns f(value), otherwise returns None.

Examples

Option.Some(3).flatMap((x) => (x > 2 ? Option.None() : Option.Some(2)));
// Option.None

Option.Some(1).flatMap((x) => (x > 2 ? Option.None() : Option.Some(2)));
// Option.Some<2>

option.flatMap((value) => value.optionalProperty);
// Option<optionalProperty>

.filter(f)

Option<A>.filter(f: (value: A) => boolean): Option<A>

If the option is Some(value) and that f(value) is true``, returns Some(value), otherwise returns None`.

Examples

Option.Some(3).filter((x) => x > 2);
// Option.Some(3)

Option.Some(1).filter((x) => x > 2);
// Option.None

.getWithDefault(defaultValue)

Option<A>.getWithDefault(defaultValue: A): A

If the option is Some(value) returns value, otherwise returns defaultValue.

Examples

Option.Some(2).getWithDefault(1);
// 2

Option.None().getWithDefault(1);
// 1

.get()

Option<A>.get(): A

Returns the value contained in Some(value). Only usable within a isSome() check.

Examples

const value = option.get();
// does not compile

if (option.isSome()) {
  const value = option.get();
  // value
}

.isSome()

Option<A>.isSome(): boolean

Type guard. Checks if the option is Some(value)

Examples

Option.Some(2).isSome();
// true

Option.None().isSome();
// false

if (option.isSome()) {
  const value = option.get();
}

.isNone()

Option<A>.isNone(): boolean

Type guard. Checks if the option is None

Examples

Option.Some(2).isNone();
// false

Option.None().isNone();
// true

.toNull()

Option<A>.toNull(): A | null

Returns null if the option is None, returns the value otherwise

Examples

Option.Some(2).toNull();
// 2

Option.None().toNull();
// null

.toUndefined()

Option<A>.toUndefined(): A | undefined

Returns undefined if the option is None, returns the value otherwise

Examples

Option.Some(2).toUndefined();
// 2

Option.None().toUndefined();
// undefined

.toResult(errorWhenNone)

Option<A>.toResult<E>(valueWhenNone: E): Result<A, E>

Returns Ok if the option is Some, returns Error otherwise

Examples

const a = Option.Some(1).toResult("NotFound");
// Ok<1>

const b = Option.None().toResult("NotFound");
// Error<"NotFound">

.match()

Option<A>.match<B>(config: {
  Some: (value: A) => B;
  None: () => B;
}): B

Match the option state

Examples

const valueToDisplay = option.match({
  Some: (value) => value,
  None: () => "No value",
});
// value | "No value"

.tap(func)

Option<A>.tap(func: (option: Option<A>) => unknown): Option<A>

Executes func with option, and returns option. Useful for logging and debugging.

Examples

option.tap(console.log).map((x) => x * 2);

Statics

Option.isOption(value)

isOption(value: unknown): boolean

Type guard, checks if the provided value is an option.

Examples

Option.isOption(Option.Some(1));
// true

Option.isOption([]);
// false

Option.all(options)

all(options: Array<Option<A>>): Option<Array<A>>

Turns an "array of options of value" into a "option of array of value".

Examples

Option.all([Option.Some(1), Option.Some(2), Option.Some(3)]);
// Some([1, 2, 3])

Option.all([Option.None(), Option.Some(2), Option.Some(3)]);
// None

Option.allFromDict(options)

allFromDict(options: Dict<Option<A>>): Option<Dict<A>>

Turns a "dict of options of value" into a "option of dict of value".

Examples

Option.allFromDict({ a: Option.Some(1), b: Option.Some(2), c: Option.Some(3) });
// Some({a: 1, b: 2, c: 3})

Option.allFromDict({ a: Option.None(), b: Option.Some(2), c: Option.Some(3) });
// None

TS Pattern interop

Examples

import { match, P } from "ts-pattern";
import { Option } from "@swan-io/boxed";

match(myOption)
  .with(Option.P.Some(P.select()), (value) => console.log(value))
  .with(Option.P.None, () => "No value")
  .exhaustive();

Cheatsheet

Method	Input	Function input	Function output	Returned value
map	Some(x)	x	y	Some(y)
map	None()	not provided	not executed	None()
flatMap	Some(x)	x	Some(y)	Some(y)
flatMap	Some(x)	x	None()	None()
flatMap	None()	not provided	not executed	None()