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Javascript – Map type definition where value corresponds to key

How can I define a map where each key has a certain type assigned ?

This is the type def:

type Data = {
  articles: Article[],
  comments: Comment[],
  ...
}

And the map def has to accept only keys from that type, but the values have to match the keys:

type MyMap = <K extends keyof Data>Map<K, Data[K]>; // <- doesn't work
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2

Answers


  1. 0

    You can make a generic type, that uses a distributed conditional type

    type MyMap<O, K extends keyof O = keyof O> = K extends any ? Map<K, O[K]> : never;

    This builds a union of Map types for each key (K) from Data and with the associated value type (O[K]):

    type MyDataMap = MyMap<Data>;
// Map<"articles", Article[]> | Map<"comments", Comment[]>

    To break it down a little, the MyMap<Data> is the same as MyMap<Data, keyof Data> (due to the default on the second generic type). keyof Data can also be written as a union type of "articles" | "comments":

    MyMap<Data>
// "unfurls" to 
MyMap<Data, "articles" | "comments">
// O--^^^^   ^^^^^^^^^^^^^^^^^^^^^----- K

    The conditional type of K extends any is then applied to each member of the "articles" | "comments" union. So MyMap distributes on each element, effectively giving:

    MyMap<Data, "articles"> | MyMap<Data, "comments">

    When ultimately gives:

    Map<"articles", Article[]> | Map<"comments", Comment[]>
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  2. 0

    I think what you’re looking for is something like the following:

    interface StrictMap<T extends Record<string, any>>
  extends Map<keyof T, T[keyof T]> {
  clear(): void;
  delete(key: keyof T): boolean;
  get<K extends keyof T>(key: K): T[K] | undefined;
  has(key: keyof T): boolean;
  set<K extends keyof T>(key: K, value: T[K]): this;
  readonly size: number;
}

    This is a copy of the Map type, but I have changed the generics to accept only an object-like value (which will retain the exact key-value mappings).

    I have also overridden most of the methods so that the types are stricter. The only one I have left out is forEach which uses the existing Map.forEach as our new type extends the Map<K, V> type.

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