TypeScript interfaces support a feature called declaration merging. An example use case of that would be something like the node JS framework called express. The base request type supported by express has a few properties, for example, the body property. Now within our application code, when we handle an express request, we will have access to any of the properties that are declared on the request type. For example, we have access to this body property. Now declaration merging allows a middleware, for example, adjacent middleware to add additional properties

to the request interface. TypeScript is going to merge the two declarations of the interface into a single type that has both of these properties. So in our code, simply by importing the JSON middleware, you will get access to the JSON property on instances of the request interface. Declaration merging is not something that is supported, but type aliases. So if you are working with an API that requires such seamless structural extension, you will have to use an interface.