Can I write a cast operator in Typescript?

Question

I've used JSWEET to transpile a large Java project. It has converted types like Enumeration<Object> directly to Enumeration<any> in TypeScript.

In the Java world it was possible to assign an array Object[] to an Enumeration<Object>.

In Typescript I get: Type 'any[]' is not assignable to type 'Enumeration<any>'

Is there a way to extend Enumeration<Object> OR any[] in such a way that TypeScript will allow this assignment to occur?

Answer 1

You can cast in two different ways:

var doSomething = function <T>(a: Enumeration<T>) { };
var x: Object[] = [];

// cast using "<>""
doSomething(<Enumeration<Object>>x);

// cast using "as"
doSomething(x as Enumeration<Object>);

But your Enumeration should probably extend Array:

interface Enumeration<T> extends Array<T> {
    // ...
}

Update

class Enumeration<T> extends Array<T> {
    private _cursor: number;
    public constructor(...elements: T[]) {
        super(...elements);
        this._cursor = 0;
    }
    public nextElement() {
        this._cursor = this._cursor + 1;
        return this.values[this._cursor];
    };
    public hasMoreElements() {
        return this._cursor < this.length - 1;
    }
    public static fromArray<T>(arr: T[]) {
        return new Enumeration<T>(...arr);
    }
}

interface User {
    name: string;
}

let a: Array<User> = [{ name: "user1" }, { name: "user2" }];
let b: Enumeration<User> = new Enumeration({ name: "user1" }, { name: "user2" });

a = b; // OK: Enumeration<User> is assignable to Array<User>
b = a; // Error: Array<User> is not assignable to Enumeration<User>

Maybe you can add a method fromArray to the enumeration class:

var doSomething = function <T>(a: Enumeration<T>) { };
doSomething(Enumeration.fromArray<User>(a));