ts-union-tools

ts-union-tools is a set of minimal utilities to work with union types and tagged unions (a.k.a. discriminated unions) in TypeScript. The main API is pattern matching utility functions called match and matchOn to do type-safe pattern-match against unions and tagged unions elegantly without if-else or switch. It also provides useful type utilities to work with tagged union types.

Features

• No opaque objects / classes. It just provides simple utility functions and type utilities to work seamlessly with existing union types in your codebase such as type Person = {role: 'Dev', language: string} | {role: 'UX', tool: string} | 'Sales'
• Each utility function is overloaded with data-last curried version, which can be nicely used with other functional composition utilities like flow in Lodash or pipe in Ramda

How to use

Make sure you run npm install ts-union-tools to make it available in your project.

matchOn function for pattern-match against tagged unions

matchOn does pattern-match against tagged-unions, even mixed with raw literals like 'Sales' in the following example. You can use any name (e.g. kind, type and role) for the name of the "tag" in tagged unions by specifying the first argument of matchOn.

import { matchOn } from 'ts-union-tools'

type Person =
  | { role: 'Dev'; language: string }
  | { role: 'UX'; tool: string }
  | 'Sales'
  | 'PM'

const person: Person =
  Math.random() > 0.5
    ? {
        role: 'Dev',
        language: 'TypeScript',
      }
    : Math.random() > 0.5
    ? {
        role: 'UX',
        tool: 'Photoshop',
      }
    : Math.random() > 0.5
    ? 'Sales'
    : 'PM'

const message = matchOn('role', person, {
  Dev: (dev) => `I am a dev using ${dev.language}`,
  UX: (ux) => `I am a UI/UX designer using ${ux.tool}`,
  Sales: () => 'I am a salesperson',
  // "_" (underscore) can be used as catch-all case
  _: () => 'I have some other role',
})

console.log(message)

match function for pattern-match against simple unions

When a union type does not contain "tagged union", it does not make sense to specify the name of "tag" (e.g. kind, type and role) In that case, we can use a simpler function, match.

import { match } from 'ts-union-tools'

type Person = 'Dev' | 'UX' | 'Sales' | 'PM'

const person: Person =
  Math.random() > 0.5
    ? 'Dev'
    : Math.random() > 0.5
    ? 'UX'
    : Math.random() > 0.5
    ? 'Sales'
    : 'PM'

const message = match(person, {
  Dev: () => 'I am a dev',
  UX: () => 'I am a UI/UX designer',
  Sales: () => 'I am a salesperson',
  PM: () => 'I have some other role',
})

console.log(message)

Type utilities (to be documented)

• GetUnionTags<UnionType>
• TaggedUnionExtract<UnionType>
• TaggedUnionExclude<UnionType>

Why not just use switch or if-else?

TBD

License

See LICENSE

Contributions

See CONTRIBUTING