Capture the type of an array so we can map it into an array of union types

Question

class Custom1 { }

class Custom2 { }

class Custom3 { }

class Custom4 { }

class Foo { 
  custom1s: Custom1[];
  custom2s: Custom2[];
  custom3: Custom3;
  custom4: Custom4;
}

type SomeMapping<T> = {
  [P in keyof T]
  : T[P] extends any[] ? object[]
  : (T[P] | object);
}

type FooMapped = SomeMapping<Foo>;

The actual typing is this:

type FooMapped = { 
  custom1s: object[]; 
  custom2s: object[]; 
  custom3: object | Custom3; 
  custom4: object | Custom4; 
}

What I would like this:

type FooMapped = { 
  custom1s: (object | Custom1)[]; 
  custom2s: (object | Custom2)[]; 
  custom3: object | Custom3; 
  custom4: object | Custom4; 
}

How do we capture the type of the array so we can turn it into a union?

Answer 1

To have type

type FooMapped = { 
  custom1s: Custom1[]; 
  custom2s: Custom2[]; 
  custom3: object | Custom3; 
  custom4: object | Custom4; 
}

you should do this

type SomeMapping<T> = {
  [P in keyof T]
  : T[P] extends (infer U)[] ? U[]
  : (T[P] | object);
}

and to achieve this type

type FooMapped = { 
  custom1s: (object | Custom1)[]; 
  custom2s: (object | Custom2)[]; 
  custom3: object | Custom3; 
  custom4: object | Custom4; 
}

do this

type SomeMapping<T> = {
  [P in keyof T]
  : T[P] extends (infer U)[] ? (U | object)[]
  : (T[P] | object);
}

Reference: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html (section "Conditional types")