Hybrid Types

Hybrid Types

As we mentioned earlier, interfaces can describe the rich types present in real world JavaScript. Because of JavaScript’s dynamic and flexible nature, you may occasionally encounter an object that works as a combination of some of the types described above.

One such example is an object that acts as both a function and an object, with additional properties:

interface Counter {
  (start: number): string;
  interval: number;
  reset(): void;
}

function getCounter(): Counter {
  let counter = <Counter>function (start: number) { };
  counter.interval = 123;
  counter.reset = function () { };
  return counter;
}

let c = getCounter();
c(10);
c.reset();
c.interval = 5.0;

When interacting with 3rd-party JavaScript, you may need to use patterns like the above to fully describe the shape of the type.