davejoem - 9 months ago 71

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

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> {};
let a: Props = {};
```

Which will do the same