typescript - How to create a Partial-like that requires a single property to be set

Question

We have a structure that is like the following:

export type LinkRestSource = {
    model: string;
    rel?: string;
    title?: string;
} | {
    model?: string;
    rel: string;
    title?: string;
} | {
    model?: string;
    rel?: string;
    title: string;
};

Which is almost the same as saying

type LinkRestSource = Partial<{model: string, rel: string, title: string}>

Except that this will allow an empty object to be passed in whereas the initial type requires one of the properties to be passed in

How can I create a generic like Partial, but that behaves like my structure above?

Answer 1

I think I have a solution for you. You're looking for something that takes a type T and produces a related type which contains at least one property from T. That is, it's like Partial<T> but excludes the empty object.

If so, here it is:

type AtLeastOne<T, U = {[K in keyof T]: Pick<T, K> }> = Partial<T> & U[keyof U]

To dissect it: first of all, AtLeastOne<T> is Partial<T> intersected with something. U[keyof U] means that it's the union of all property values of U. And I've defined (the default value of) U to be a mapped type where each property of T is mapped to Pick<T, K>, a single-property type for the key K. (For example, Pick<{foo: string, bar: number},'foo'> is equivalent to {foo: string}... it "picks" the 'foo' property from the original type.) Meaning that U[keyof U] in this case is the union of all possible single-property types from T.

Hmm, that might be confusing. Let's see step-by-step how it operates on the following concrete type:

type FullLinkRestSource = {
  model: string;
  rel: string;
  title: string;
}

type LinkRestSource = AtLeastOne<FullLinkRestSource>

That expands to

type LinkRestSource = AtLeastOne<FullLinkRestSource, {
  [K in keyof FullLinkRestSource]: Pick<FullLinkRestSource, K>
}>

or

type LinkRestSource = AtLeastOne<FullLinkRestSource, {
  model: Pick<FullLinkRestSource, 'model'>,
  rel: Pick<FullLinkRestSource, 'rel'>,
  title: Pick<FullLinkRestSource, 'title'>
}>

or

type LinkRestSource = AtLeastOne<FullLinkRestSource, {
  model: {model: string},
  rel: {rel: string},
  title: {title: string}>
}>

or

type LinkRestSource = Partial<FullLinkRestSource> & {
  model: {model: string},
  rel: {rel: string},
  title: {title: string}>
}[keyof {
  model: {model: string},
  rel: {rel: string},
  title: {title: string}>
}]

or

type LinkRestSource = Partial<FullLinkRestSource> & {
  model: {model: string},
  rel: {rel: string},
  title: {title: string}>
}['model' | 'rel' | 'title']

or

type LinkRestSource = Partial<FullLinkRestSource> &
  ({model: string} | {rel: string} | {title: string})

or

type LinkRestSource = {model?: string, rel?: string, title?: string} & 
  ({model: string} | {rel: string} | {title: string})

or

type LinkRestSource = { model: string, rel?: string, title?: string } 
  | {model?: string, rel: string, title?: string} 
  | {model?: string, rel?: string, title: string}

which is, I think, what you want.

You can test it out:

const okay0: LinkRestSource = { model: 'a', rel: 'b', title: 'c' }
const okay1: LinkRestSource = { model: 'a', rel: 'b' }
const okay2: LinkRestSource = { model: 'a' }
const okay3: LinkRestSource = { rel: 'b' }
const okay4: LinkRestSource = { title: 'c' }

const error0: LinkRestSource = {} // missing property
const error1: LinkRestSource = { model: 'a', titel: 'c' } // excess property on string literal

So, does that work for you? Good luck!