One issue to consider (besides the common advice not to extend native object prototypes) is that when defining a generic interface or type, the type parameter must be a plain name and cannot be based on another type. For example:
interface Foo<T, U[]> {} // error
If you need the second type parameter to be an array, you can use a generic constraint instead:
interface Foo<T, U extends any[]> {} // okay
Another challenge arises when attempting to merge declarations of generic types if altering the type parameters or constraints. You are restricted from changing predefined types like Map<K, V>, resulting in having to maintain it as is.
Introducing a new method that requires V to be an array poses unique constraints within your declaration which might seem limiting. However, utilizing a TypeScript feature known as a this parameter offers a viable solution. By adding this parameter at the start of the function or method signature, you can enforce proper usage without risking invalid calls.
An example implementation to clarify:
// no error
declare global {
interface Map<K, V> {
SetInArray<V>(this: Map<K, V[]>, key: K, value: V): boolean;
}
}
Map.prototype.SetInArray = function <K, V>(this: Map<K, V[]>, key: K, value: V): boolean {
let isNew: boolean = false;
if (this.has(key) == false) {
this.set(key, []);
isNew = true;
}
let items: V[] = this.get(key)!;
items.push(value);
return isNew;
};
A demonstration follows:
const numberArrayMap = new Map<string, number[]>();
numberArrayMap.set("a", [1, 2, 3])
numberArrayMap.SetInArray("a", 4); // okay
const numberMap = new Map<string, number>();
numberMap.set("a", 4)
numberMap.SetInArray("a", 4); // error
// The 'this' context of type 'Map<string, number>' is not assignable
// to method's 'this' of type 'Map<string, number[]>'.
// Type 'number' is not assignable to type 'number[]'.
This behavior provides the desired functionality where you can call SetInArray() on numberArrayMap, but not on
numberMap</code due to mismatching <code>this
contexts.
Hopefully, this information proves helpful for your endeavors. Best of luck!