Type inference

You don’t have to explicitly type everything, TypeScript will use the type of the initial value of a variable. It works for more complex types too.

const [season, setSeason] = React.useState({name:"Christmas", color:"red"});//in order to set the state you can do this:setSeason({name:"Easter", color: "yellow"});

If you realise you need the type in retrospect, you can can go back and declare the type, or you can use typeof to declare the type based on the inference.

type Season = typeof season;

Union types

When I started out with typescript and React I was mostly familiar with typed languages through Java so this was the first time I came across union types. If you haven’t come across it before it basically allows you to say that a variable can be one of several types. It’s useful if you want to avoid type inference but still want a variable to be able to have more than one type. For instance if you want to initialize a state with a null value

const [season, setSeason] = React.useState<Season | null>(null);

Discriminated Unions

In the same vein as the tip above, here’s another tip for typing which incorporates union types. If you make a union type of several types that have one common literal type that you can use to tell the types apart you have what’s called a discriminated union type. Useful if you want your components accept different sets of props.

type NeatTrickProps = | { hasTheme: true; season: 'christmas' | 'easter'; } | { hasTheme: false; };export const NeatTrickBanner: FunctionComponent<NeatTrickProps> = props => { if (props.hasTheme) { return seasonBanner(props.season); } return ( <div> <h1>No reason to celebrate</h1> </div> ); };}

So that’s a few features I really wish I was familiar with when I started out using react and Typescript. If you want to read more I highly recommend these cheatsheets and the official typescript docs

Happy holidays :-)