Add Public<T>

[ssonne]

Problem: a class with private members cannot be mocked without bypassing the typechecker.

Solution: Add a type Public<T> that maps only the public members. This makes it easy to get a class's public interface for dependency injection and mocking.

type Public<T> = { [P in keyof T]: T[P] };

export class Foo {
    private _priv = 1;

    public getStr():string {
        return 'abc';
    }
}

// Type '{ getStr(): string; }' is not assignable to type 'Foo'.
//  Property '_priv' is missing in type '{ getStr(): string; }'.
const naiveFake: Foo = {
    getStr(): string {
        return 'def';
    }
}

export type IFoo = Public<Foo>;

// Works!
const fake: IFoo = {
    getStr(): string {
        return 'def';
    }
}

See this comment:
microsoft/TypeScript#18499 (comment)

[pelotom]

Thanks!