Declare a string type with a set of possible values

I know that the following works:

interface Travel {
  travelClass: 'economy' | 'business' | 'first';
  username?: String;
}

Now, I want to extract the travelClass so that I can reuse it at other places, something like this:-

type TravelClass 'economy' | 'business' | 'first'

However, it is not compiling.

>Solution :

type requires the use of a =.

type TravelClass  = 'economy' | 'business' | 'first'