Typescript const assertions in practice

With Typescript 3.4 we got const assertions. Armed with this knowledge we can take a look at the following demo:

Fixating type with const assertion

First we declare our type : Genres that will be used to make a stronger type than string e.g. 'comedy' | 'horror'. In the demo above we can see that when we create an object (without declaring its type) firstBook, Typescript will infer genre to string since no other information is provided.

By attempting to use this object in our createCover() fn Typescript will yell at us since we are expecting an object whose genre is of type Genres and we are giving it a string.

Const assertion to the rescue

in bestCaseFirstBook we assert genre: 'comedy' as const thus fixating the type of genre to ‘comedy’ and not string. Typescript can now safely assume that type ‘comedy’ is found in Genres type and all is well.