Typescript 从条件类型的联合子集中推断值

Question

I'm looking for a little help on type inference to narrow a union in TypeScript based on a conditional type.

Our API takes a set of parameters and validates them. As part of the validation process, it normalises all the values to make it easier for downstream processing. One of the parameters can be either an array of strings or an array of array of strings. When normalised that value is always converted to an array of array of strings.

To avoid having to write out each validated type explicitly (and maintain them) I am trying to infer the validated params from the incoming params (which have the same keys) with the resolved value. For example:

type IncomingParams = {
   names: string[] | Array<string[]> | undefined
   ages: string[] | undefined
}

// Post validation the object is the following
type Validated = {
   names: Array<string[]>
   ages: string[]

}

This is what I came up with but it doesn't ensure that when there is the union, it always resolves to an array of array.

type ArrayUnion<T> = T[] | T[][];

type Params = {
   names: ArrayUnion<string>
   ages: string[]
}

type ValidatedParams<P> = {
    [K in keyof P]: P[K] extends ArrayUnion<infer U>
        ? U[][]
        : NonNullable<P[K]>;
};

const validated: ValidatedParams<Params> = {} as any;

validated.names // This should be string[][]
validated.ages // This should be string[]

It may be that Typescript cannot differentiate the union, so one option I've explored is to use & { readonly _sym: unique symbol } to uniquely identify ArrayUnion , although this approach hasn't worked either.

I'd appreciate any help in advance.

TypeScript playground

Answer 1

It seems there the undefined being added caused some issues. Here is a solution working in the TS playground

https://www.typescriptlang.org/play?experimentalDecorators=false&emitDecoratorMetadata=false&target=2&module=1#code/PTAEHUHsCcGsEsB2BzUBnSAbArgF3pIgFBG4CeADgKagCC00AhmQKqIGIA8AKgHygBeUAG1uoAD6huwgLoAaUADplACmkyJU2bICUGydkQATKgDMkVIyXLVQABUZMAtmkGgA3kVDfEjJ1TQALjoGZjYOTjRcaCRkXi9vRmQA4KiYlFk5BNBIXAALKmhg+iZWdkJI6Nj4gF9rShoANUZMeCNGXEsHZzROO34hT29vYQBpUCRQWCoySFN7GWC7MY0qAA9O41cSsPKuJFNC0BZ44bPQAH5QADlCa+xMTEYAI0wqThPtGWzz4NvEe6PF5vPorXgAbiINUhRAAxoQoqAAG4tNodSzBZqtdqdIzdPy9fEuAYeGqgRiuRiIMgwlHY9FGRS+fyuUAgKR5eCuNB5SAPIygZ40NKxL5EOlo3GKJIBbzs7ic7m8-mC4VVDLfIjsgDChFh0CoHQmTgomH8iFwHQ4cIRuFAFEcBKWjpcbiGwxlQREACIsEZvfIfsyUj6AFZ+AIBrJnXIFIqgQwmcyISxQkjwxCIh09ABMzp6bp+nuCwl9mH9gbOwa9wlL4ZZUbDkDyiG9Cm97SRVADleGscKJbrEbQjbrzdb7c73fkPopFcDdSAA