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Filter using a type assertion

@RiccardoOdone•Suggest a tip•

When you filter an array of union types, make sure it gets reflected in the type.

type Admin = { _kind: 'admin', adminId: number }
type Member = { _kind: 'member', userId: number }
type User = Admin | Member

const onlyAdmins = users.filter(x => x._kind === 'admin')
//    ^? const onlyAdmins: User[]
//    ⛔️ Should be Admin[]

const isAdmin = (user: User): user is Admin =>
  user._kind === 'admin'
const onlyAdmins = users.filter(isAdmin)
//    ^? const onlyAdmins: Admin[] ✅
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