Typescript Generics: return instance of class parameter

Question

I have a data store and want to create a method to load data from the store. The store contains different types (classes) of data. Let's say my store contains (among other) data of type Author. I want to load author with id 1:

const author1 = store.loadById(Author, 1);

Now how do I have to use generics to get the TS compiler know that author1 is an instance of Author?

I now have

public loadById<T>(entityClass: T, id: number): T {
        const entity;
        // logic to load the entity ...
        return entity;
    }

but that is wrong because now the TSC thinks my method returns a entityClass rather than an instance of the entityClass. So how do I have to specify the return type of the method to get author1 to be an instance of Author?

Answer 1

You are passing in the class Author to the method, not an instance of the Author class, so the parameter needs to be a constructor signature:

public loadById<T>(entityClass: new () => T, id: number): T {
    const entity = new entityClass();
    // logic to load the entity ...
    return entity;
}
const author1 = store.loadById(Author, 1); // will be of type Author

Or if you have argument to the constructor, you can specify those in the signature:

public loadById<T>(entityClass: new (data: any) => T, id: number): T {
    const entity = new entityClass(null as any); // pass data
    // logic to load the entity ...
    return entity;
}

Answer 2

For those who want to pass a class and return an instance of the class

function newInstance<T extends new () => InstanceType<T>>(TheClass: T): InstanceType<T> {
    return new TheClass();
}