I have a method getUniqueId which accepts two kinds of interfaces below, which will return the uniqueId depending which interface type passed:

interface ReadOnlyInfo {

    itemId: string;

    ....

}



interface EditInfo {

    formId: number; 

    ....

}



function getUniqueId (info: ReadOnlyInfo | EditInfo) {

   if (info instanceof ReadOnlyInfo) {

       return info.itemId;

   }

   return info.formId;

}

I am wondering if this is a good practice to use instanceof here. My concern is that I might have many other methods similar to getUniqueId, which accept "ReadOnlyInfo | EditInfo" type as well, so I have to repeat "ReadOnlyInfo | EditInfo" everywhere.

Instead of doing that, I also tried using type:

type CommonInfo = | ReadOnlyInfo | EditInfo;

so, that I can save some code (just do CommondInfo), like below, but that way, I cannot tell which type CommonInfo is, instanceof no longer works and gives me compiler errors.

function getUniqueId (info: CommonInfo) {

     if (info instanceof ReadOnlyInfo) {

           return info.itemId;

       }

       return info.formId;   

}

So, I am wondering what would be the best practice to design interface/methods in this scenario. Thanks in advance!