A method to narrow down an array of discriminated unions

narrowingDiscriminatedUnionArray.ts

type One = { one: 'one'; t: 'one' }
type Two = { two: 2; t: 'two' }

type OneOrTwo = One | Two

const rows: OneOrTwo[] = []

const getOneOrTwo = <T extends OneOrTwo['t']>(t: T) => {
 return  rows.find((obj): obj is Extract<OneOrTwo, {t: T}> =>  obj.t === t)
}

let one = getOneOrTwo('one')

if (one) {
  one.one == 'one'
}

let two = getOneOrTwo('two')

if (two) {
  two.two == 2
}

Kudos to this stack overflow post.

.filter would also work