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HTML / CSS / JS

// Callable interface — function with properties
interface Logger {
    (message: string): void;       // call signature
    level: "info" | "warn" | "error";
    prefix: string;
}

const appLog = createLogger("App");
appLog("Server started");
appLog.level = "warn";
appLog("Memory usage high");

// Construct signature — accept a class constructor
interface Constructable<T> {
    new (id: number, name: string): T;
}

interface Entity {
    id: number;
    name: string;
}

function createEntity<T extends Entity>(
    Ctor: Constructable<T>,
    id: number,
    name: string
): T {
    return new Ctor(id, name);
}

class Product implements Entity {
    constructor(public id: number, public name: string) {}
    describe(): string { return `Product #${this.id}: ${this.name}`; }
}

const p = createEntity(Product, 101, "Widget");
console.log(p.describe());