How can I modify the join() function of an Array<MyType> in Typescript to instead return MyType instead of a string?

I am working with a specialized string type called 

MyType = string & { __brand: 'mytype' }
.

Is there a way to define an override for the Array.join method specifically for arrays of type Array<MyType> so that it returns MyType instead of string?

Additionally, I would like the solution to be generic and capable of handling any type that extends, such as Mergeable, allowing it to join and create a new instance of itself.

type Mergeable = string & { __mergeable: true }

type MyType = Mergeable & { __brand: 'MyType' }

const arr: MyType[] = ['a', 'b', 'c'] as any
const merged: MyType = arr.join(', ' as MyType)
typescript

Answer №1

When working with TypeScript, interfaces are combined when declared multiple times. This allows you to include an additional declaration of the join() method, which will act as an overload of the default Array<T>.join().

The concept of merging interfaces in TypeScript is explained here. It involves mechanically joining the members of both declarations into a single interface with the same name.

In the case of function members, each member with the same name is considered an overload of the same function.

type MyType = string & { __brand: 'mytype' }

interface Array<T> {
    join(this: Array<MyType>, separator?: MyType): MyType
}

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