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 example 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

That's called Type Assertion or casting.

These are the same:

let square = <Square>{};
let square = {} as Square;

Example:

interface Props {
    x: number;
    y: number;
    name: string;
}

let a = {};
a.x = 3; // error: Property 'x' does not exist on type `{}`

So you can do:

let a = {} as Props;
a.x = 3;

Or:

let a = <Props> {};

Which will do the same

Answer 2

This is called Type Assertion.

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