Transform union type to intersection type

Question

Is there a way to transform a union type into an intersection type :

type FunctionUnion = () => void | (p: string) => void
type FunctionIntersection =          


        

Answer 1

There is also a very related problem when you would like an intersection of several types, but not necessarily convert unions to intersections. There is just no way to get right to intersections without resorting to temporary unions!

The problem is that types we would like to get an intersection of might have unions inside, which will be converted to intersections too. Guards to the rescue:

// union to intersection converter by @jcalz
// Intersect<{ a: 1 } | { b: 2 }> = { a: 1 } & { b: 2 }
type Intersect = (T extends any ? ((x: T) => 0) : never) extends ((x: infer R) => 0) ? R : never

// get keys of tuple
// TupleKeys<[string, string, string]> = 0 | 1 | 2
type TupleKeys = Exclude

// apply { foo: ... } to every type in tuple
// Foo<[1, 2]> = { 0: { foo: 1 }, 1: { foo: 2 } }
type Foo = {
    [K in TupleKeys]: {foo: T[K]}
}

// get union of field types of an object (another answer by @jcalz again, I guess)
// Values<{ a: string, b: number }> = string | number
type Values = T[keyof T]

// TS won't believe the result will always have a field "foo"
// so we have to check for it with a conditional first
type Unfoo = T extends { foo: any } ? T["foo"] : never

// combine three helpers to get an intersection of all the item types
type IntersectItems = Unfoo>>>

type Test = [
    { a: 1 } | { b: 2 },
    { c: 3 },
]

// this is what we wanted
type X = IntersectItems // { a: 1, c: 3 } | { b: 2, c: 3 }

// this is not what we wanted
type Y = Intersect // { a: 1, b: 2, c: 3 }

The execution in the given example goes like this

IntersectItems<[{ a: 1 } | { b: 2 }, { c: 3 }]> =
Unfoo>>> =
Unfoo>> =
Unfoo> =
Unfoo<(({ foo: { a: 1 } | { b: 2 } } | { foo: { c: 3 } }) extends any ? ((x: T) => 0) : never) extends ((x: infer R) => 0) ? R : never> =
Unfoo<(({ foo: { a: 1 } | { b: 2 } } extends any ? ((x: T) => 0) : never) | ({ foo: { c: 3 } } extends any ? ((x: T) => 0) : never)) extends ((x: infer R) => 0) ? R : never> =
Unfoo<(((x: { foo: { a: 1 } | { b: 2 } }) => 0) | ((x: { foo: { c: 3 } }) => 0)) extends ((x: infer R) => 0) ? R : never> =
Unfoo<{ foo: { a: 1 } | { b: 2 } } & { foo: { c: 3 } }> =
({ foo: { a: 1 } | { b: 2 } } & { foo: { c: 3 } })["foo"] =
({ a: 1 } | { b: 2 }) & { c: 3 } =
{ a: 1 } & { c: 3 } | { b: 2 } & { c: 3 }

Hopefully this also shows some other useful techniques.