option

options for node.js

An implementation of the option type, sometimes known as the maybe type.

An instance of an option type is an optional value. Either it's none, or an instance of Some:

var option = require("option");

var some = option.some("Bob");
var none = option.none;

A function that returns an optional string isn't that different from a function that returns a string or null. The advantage over null is that options provide a number of functions that help with manipulating optional values.

    function greet(user) {
        return "Hello " + user.name().valueOrElse("Anonymous");
    }

Methods

isNone() and isSome()

• some(value).isNone() returns false
• some(value).isSome() returns true
• none.isNone() returns true
• none.isSome() returns false

value()

• some(value).value() returns value
• none.value() throws an error

map(func)

• some(value).map(func) returns some(func(value))
• none.map(func) returns none

flatMap(func)

Conventionally used when func returns another option.

• some(value).flatMap(func) returns func(value)
• none.flatMap(func) returns none

filter(predicate)

• some(value).filter(predicate) returns:
  • some(value) if predicate(value) === true
  • none if predicate(value) === false
• none.filter(predicate) returns none

toArray()

• some(value).toArray() returns [some]
• none.toArray() returns []

orElse(other)

If other is a function (other conventionally returning another option):

• some(value).orElse(other) returns some(value)
• none.orElse(other) returns other()

If other is not a function (other conventionally being another option):

• some(value).orElse(other) returns some(value)
• none.orElse(other) returns other

valueOrElse(other)

If other is a function:

• some(value).valueOrElse(other) returns value
• none.valueOrElse(other) returns other()

If other is not a function:

• some(value).valueOrElse(other) returns value
• none.valueOrElse(other) returns other

Functions

option.isOption(value)

• option.isOption(value) returns true if value is option.none or option.some(x).

option.fromNullable(value)

• If value is null or undefined, option.fromNullable(value) returns option.none.
• Otherwise, returns option.some(value). For instance, option.fromNullable(5) returns option.some(5).

Installation

npm install option