Class Decorator

class-decorator.ts

export function logClass(target: Function) {
    // save a reference to the original constructor
    const original = target;

    // a utility function to generate instances of a class
    function construct(constructor, args) {
        const c: any = function () {
            return constructor.apply(this, args);
        }
        c.prototype = constructor.prototype;
        return new c();
    }

    // the new constructor behaviour
    const f: any = function (...args) {
        console.log(`New: ${original['name']} is created`);
        return construct(original, args);
    }

    // copy prototype so intanceof operator still works
    f.prototype = original.prototype;

    // return new constructor (will override original)
    return f;
}

@logClass
class Employee {

}

let emp = new Employee();
console.log('emp instanceof Employee');
console.log(emp instanceof Employee);