折叠一个有区别的联合——从联合中派生出一个包含所有可能的键值组合的伞型

Question

我有一个受歧视的工会，例如：

type Union = { a: "foo", b: string, c: number } | {a: "bar", b: boolean }

我需要派生一个包含所有潜在属性的类型，并分配可能在 Union 的任何成员上找到的类型，即使只在某些成员上定义 - 在我的示例中：

type CollapsedUnion = { 
  a: "foo" | "bar", 
  b: string | boolean, 
  c: number | undefined 
}

如何创建派生此类折叠联合的泛型？
我需要一个支持任意大小联合的泛型。

可以实现类似的行为 as a byproduct by using native Omit utility，但对我来说不幸的是，它遗漏了每个联合成员都不存在的属性（不将它们计入 undefined 或通过 ?).

Answer 1

可行；下面介绍了一种可能的解决方案。如果有可能以更简单的方式实现它，我会很感兴趣。我添加了注释来引导您完成代码。

// an axiliary type -- we need to postpone creating a proper union, as a tuple type can be traversed recursively
// I added additional branch to make the task a bit harder / to make sure it works in a more generic case
type ProtoUnion = [{ a: "foo", b: string, c: number }, {a: "bar", b: boolean }, { c: string }]

// an axiliary type to recover proper Union
type CollapseToUnion<T extends Record<string, any>[], Acc = {}> = // starting with a tuple of records and accumulator
  T extends [infer H, ...infer Rest] ? // traverse
    Rest extends Record<string, any>[] ? // if still a record present
      CollapseToUnion<Rest, (H | Acc)> : // recursive call: collapse as union
        // in other cases return accumulator 
        Acc : 
          Acc

// union recovered
type Union = CollapseToUnion<ProtoUnion>

// this type is empty, so starting with union is _impossible_ to recover all needed keys in a generic way 
type UnionKeys = keyof Union 

// this type achieves what you are asking for but only for 2 types
type MergeAsValuesUnion<A, B> = { [K in (keyof A | keyof B)]: 
    K extends keyof A ? 
      K extends keyof B ? A[K] | B[K] : 
        A[K] | undefined :
          K extends keyof B ? B[K] | undefined :
            never
  }

type OriginalUnionIntersected = MergeAsValuesUnion<ProtoUnion[0], ProtoUnion[1]>
/*
type OriginalUnionIntersected = {
    a: "foo" | "bar";
    b: string | boolean;
    c: number | undefined;
}
*/


// this is works exactly the same as CollapseToUnion, but instead of reducing with | 
// it uses MergeAsValuesUnion to reduce
type CollapseToIntersetion<T extends Record<string, any>[], Acc = {}> = T extends [infer H, ...infer Rest] ?
  Rest extends Record<string, any>[] ?
    CollapseToIntersetion<Rest, MergeAsValuesUnion<H, Acc>> 
    : Acc : Acc


const i: CollapseToIntersetion<ProtoUnion> = {
  a: 'bar', // "bar" | "foo" | undefined
  b: true, // string | boolean | undefined
  c: undefined // string | number | undefined
}

编辑：

CollapseToIntersetion 被咬掉了。以 {} 作为默认累加器开始导致值类型具有 | undefined。

// this is works exactly the same as CollapseToUnion, 
// but instead of reducing with | -- it uses MergeAsValuesUnion to reduce; 
// Acc = T[0] since Acc = {} would result in all values types unioned with undefined
type CollapseToIntersetion<T extends Record<string, any>[], Acc = T[0]> = T extends [infer H, ...infer Rest] ?
  Rest extends Record<string, any>[] ?
    CollapseToIntersetion<Rest, MergeAsValuesUnion<H, Acc>> 
    : Acc : Acc

PLAYGROUND

Answer 2

我找到了一个两个条路！

编辑：这是一个具有两个独立类型参数的解决方案。请参阅下方以了解具有单个联合类型参数的解决方案。

// The source types
type A = { a: "foo", b: string, c: number }
type B = { a: "bar", b: boolean }

// This utility lets T be indexed by any (string) key
type Indexify<T> = T & { [str: string]: undefined; }

// Where the magic happens ✨
type AllFields<T, R> = { [K in keyof (T & R) & string]: Indexify<T | R>[K] }

type Result = AllFields<A, B>
/**
 * 
 * type Result = {
 *   a: "foo" | "bar";
 *   b: string | boolean;
 *   c: number | undefined;
 * }
 */

工作原理

AllFields 是映射类型。映射类型的 'key' 部分

[K in keyof (T & R) & string]

表示 K 扩展联合 T & R 的键，这意味着它将是 T 或 [=19= 中所有键的联合].这是第一步。它确保我们正在创建一个包含所有必需键的对象。

& string 是必需的，因为它指定 K 也必须是字符串。无论如何，情况几乎总是如此，因为 JS 中的所有对象键都是字符串（偶数）——符号除外，但无论如何它们都是不同的鱼。

类型表达式

Indexify<T | R>

returns T 和 R 的联合类型，但添加了字符串索引。这意味着如果我们尝试通过 [ 索引它，TS 不会抛出错误=16=] 即使 K 在 T 或 R.

之一中不存在

最后

Indexify<T | R>[K]

意味着我们正在通过 K 索引这个 union-with-undefineds-for-string-indexes。其中，如果 K 是 T、R 或两者的键，将导致该键的值类型。

否则，它将返回到 [string]: undefined 索引并导致值为未定义。

Here's a playground link

编辑：单个通用参数的解决方案

您指定您实际上不希望它对两个类型参数起作用，而是对现有的联合类型起作用，而不管该联合中有多少成员。

付出了血汗和泪水，但我做到了。

// Magic as far as I'm concerned.
// Taken from 
type UnionToIntersection<U> =
  (U extends any ? (k: U) => void : never) extends ((k: infer I) => void) ? I : never

// This utility lets T be indexed by any key
type Indexify<T> = T & { [str: string]: undefined; }

// To make a type where all values are undefined, so that in AllUnionKeys<T>
// TS doesn't remove the keys whose values are incompatible, e.g. string & number
type UndefinedVals<T> = { [K in keyof T]: undefined }

// This returns a union of all keys present across all members of the union T
type AllUnionKeys<T> = keyof UnionToIntersection<UndefinedVals<T>>

// Where the (rest of the) magic happens ✨
type AllFields<T> = { [K in AllUnionKeys<T> & string]: Indexify<T>[K] }


// The source types
type A = { a: "foo", b: string, c: number }
type B = { a: "bar", b: boolean; }

type Union = A | B

type Result = AllFields<Union>
/**
 * 
 * type Result = {
 *   a: "foo" | "bar";
 *   b: string | boolean;
 *   c: number | undefined;
 * }
 */

我从@jcalz 的精彩回答中得到。我试图理解它，但做不到。不管怎样，我们可以把它当作一个将联合类型转换为交集类型的魔盒。这就是我们需要得到我们想要的结果的全部。

New TS playground link

Answer 3

这个基于 Aron 的回答的解决方案递归地深度折叠联合，而不仅仅是在顶层：

export type ExtractObjects<T> = Extract<T, Record<keyof any, any>>
export type ExcludeObjects<T> = Exclude<T, Record<keyof any, any>>

export type UnionToIntersection<U> = (U extends any ? (k: U) => void : never) extends (
  k: infer I
) => void
  ? I
  : never

export type Indexify<T> = T & { [str: string]: undefined }
export type AllUnionKeys<T> = keyof UnionToIntersection<{ [K in keyof T]: undefined }>
// 
export type CollapseUnionOfOnlyObjects<T extends Record<keyof any, any>> = {
  [K in AllUnionKeys<T> & string]: Indexify<T>[K]
}

type ExtractAndCollapseObjects<T> = CollapseUnionOfOnlyObjects<ExtractObjects<T>>

// recursive union collapse
export type CollapseUnion<T> = ExtractObjects<T> extends never
  ? T
  :
      | {
          [K in keyof ExtractAndCollapseObjects<T>]: CollapseUnion<ExtractAndCollapseObjects<T>[K]>
        }
      | ExcludeObjects<T>

折叠一个有区别的联合——从联合中派生出一个包含所有可能的键值组合的伞型

Collapsing a discriminated union - derive an umbrella type with all possible key-value combinations from the union

typescript

generics

union

discriminated-union

编辑：