Wrong types ?

Hi, it seems you are using & to merge two different types but that's not how it works in TypeScript, if you do A & B B properties won't overwrite A properties. If you want to do so you can do an helper type like type Merge<A, B> = Omit<A, keyof B> & B;.

This make typing non usable like so

It should say : all: [string, string, { [key: string]: string }][]; // TODO: see above comment on iterator``
Are you also sure that every single types of the object should me a string ? Key are always string sure, but values always string ? I can't put numbers ?
Shouldn't all those method be async too ? And all method is also not a method in types ?

I overwrote this type to :

type DocumentsDatabase<T, E extends object> = {
    type: "documents";
    put: (doc: E) => Promise<void>;
    del: (key: string) => Promise<void>;
    get: (key: string) => Promise<E | null>;
    iterator: (args: { amount: number }) => Promise<Iterable<[string, string, E]>>;
    query: (findFn: (doc: E) => boolean) => Promise<E[]>;
    indexBy: T;
    all: () => Promise<[string, string, E][]>;
};

Start by locating the DocumentsDatabase type definition in this TypeScript definitions repository and compare its merge, iterator, query, and all signatures with the reported usage. Resolve the intended property overriding, value types, async behavior, and tuple shapes, then verify the declarations against representative TypeScript usage or existing checks.