Create an union type from interface properties

Question

Consider the following interface

type State = {
  data: any[]
  index: number
  size: number
  count: number
  loading: boolean
  error: string | undefined
}

I need to create a type that is the union of its properties that should be like

{data: any[]} | {index: number} | {size: number} |...

So far I'm able to merge all the return types by using an indexed type access but I will get the union of the return types of the interface properties.

Answer 1

You need to use distributive-conditional-types:

type State = {
    data: any[]
    index: number
    size: number
    count: number
    loading: boolean
    error: string | undefined
}

type Split<T> = keyof T extends infer Keys // turn on distributivity
    ? (Keys extends PropertyKey
        ? (Keys extends keyof T
            ? Record<Keys, T[Keys]> // apply to each key, or { [prop in Keys]: T[Keys] }for readability
            : never)
        : never)
    : never

//  Record<"data", any[]> | Record<"index", number> | Record<"size", number> ...
type Result = Split<State>

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