What does enclosing a class in angle brackets "<>" mean in TypeScript?

Question

I am very new to TypeScript and I am loving it a lot, especially how easy it is to do OOP in Javascript. I am however stuck on trying to figure out the semantics when it comes to using angle brackets.

From their docs, I have seen several examples like

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;
}

and

interface Square extends Shape, PenStroke {
  sideLength: number;
}
  
let square = <Square>{};

I am having trouble understanding what this exactly means or the way to think of/understand it.

Could someone please explain it to me?

Answer 1

This is called Type Assertion.

You can read about it in Basarat's "TypeScript Deep Dive", or in the official TypeScript handbook.

You can also watch this YouTube video for a nice introduction.