This content originally appeared on DEV Community and was authored by Conan
Photo by Parham Moieni on Unsplash
After learning about the exciting, but sadly only Stage 1 proposal for pattern-matching in JavaScript, I felt compelled to write a library that tries to, erm, match it as closely as I could:
import { match, when, otherwise } from 'match-iz'
match(haystack)(
when(needle)(true),
when(thread)(false),
otherwise('bale')
)
Essentially, pattern-matching is if/then and switch/case with a little more declarative cowbell. ? ?
We can probe into an object:
const todosReducer = (state, action) =>
match(action)(
when({ type: 'set-vis-filter' })
(({ visFilter }) => ({
...state,
visFilter
})),
when({ type: 'add-todo' })
(({ text }) => ({
...state,
todos: [...state.todos, { text, completed: false }]
})),
otherwise(state)
)
...test against predicates as well as literals:
const defined = obj => !!obj
function AccountPage(props) {
return match(props)(
when({ error: true })(<Error {...props} />),
when({ loading: true })(<Loading />),
when({ data: defined })(<Page {...props} />),
otherwise(<Logout />)
)
}
...which is handy for dealing with server-responses:
const getJsonLength = async () =>
match(await fetch('/json'))(
when({ status: 200 })
(({ headers: { 'Content-Length': s } = {} }) => {
return `size is ${s}`
}),
when(({ status }) => status >= 500)('Server error!'),
when({ status: 404 })('JSON not found'),
when({ status: gte(400) })('Flagrant error!'),
otherwise("I didn't understand that...")
)
...and match strings against regular-expressions, passing the result into the handler:
match('1 + 2')(
when(/(?<left>\d+) \+ (?<right>\d+)/)
(({ groups: { left, right } }) =>
add(left, right)
),
when(/(?<left>\d+) \- (?<right>\d+)/)
(({ groups: { left, right } }) =>
subtract(left, right)
),
otherwise("I couldn't parse that!")
)
// 3
The only thing we can't have is else instead of otherwise. ?
The full TC39 proposal is more far-reaching because it is inspired by the advanced pattern-matching features offered by functional languages with mature implementations.
Still, while we wait for the language feature I think match-iz (along with many similar libraries) is a nice tool for constructing some of the conditional logic we use in our JavaScript.
It's not a substitute for well named intermediate variables of course, but I hope you can see its utility.
This content originally appeared on DEV Community and was authored by Conan
Conan | Sciencx (2021-08-18T21:41:59+00:00) Pattern matching = switch++. Retrieved from https://www.scien.cx/2021/08/18/pattern-matching-switch/
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