Limit properties on instantiation of Typescript object

Question

I know per this article that well typed JSON can easily be assigned to an object. For example with the class:

export class User {
    id?: number;
    username: string;

    constructor(username: string) {
        this.username = username;
    }
}

If I had a JSON formatted variable jsonUser :

const jsonUser = {
    username: bob,
};

Then I could easily say:

user: User = <User>jsonUser;

The question I have is how do I write a class, if possible, which limits the properties that class can have. For example:

const jsonUser = {
    username: bob,
    email: bob@bob.bob
};

My User class as defined above does not have email in it. Currently, it allows email to be assigned as a property to my variable of type User . I'd rather it throw an error or not assign it.

Answer 1

I thought this wouldn't be possible, but I found this interesting post in a feature request for exact types in TypeScript.

Essentially you can do this:

export class User {
    id?: number;
    username: string;

    constructor(username: string) {
        this.username = username;
    }
}

type Exact<A extends object> = A & {key: keyof A};

const jsonUser = {
    username: 'bob',
};

const jsonUser2 = {
    username: 'bob',
    id: 2
};

const jsonUser3 = {
    username: 'bob',
    email: 'bob@bob.bob'
};

const user: User = jsonUser as Exact<User>; // OK
const user2: User = jsonUser2 as Exact<User>; // OK
const user3: User = jsonUser3 as Exact<User>; // TypeScript error

The final assignment gives me the following TypeScript error:

Conversion of type '{ username: string; email: string; }' to type 'Exact<User>' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
  Type '{ username: string; email: string; }' is not comparable to type '{ key: "id" | "username"; }'.
    Property 'key' is missing in type '{ username: string; email: string; }'.

Not the most intuitive message but it does the job.

Credit for the solution goes to Rafał Łużyński (mindbrave) on GitHub.