Typescript: How to check interface type

Question

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!

Answer 1

Interfaces in Typescript just syntatic sugar for your ide. It is unpossible to check interface types like in other languages. Knowing in C# for example.

But with type guard you can recognize which interface are in use.

interface ReadOnlyInfo {
    itemId: string;
    ....
}

interface EditInfo {
    formId: number; 
    ....
}

function getUniqueId (info: ReadOnlyInfo | EditInfo) {
    if (isReadOnlyInfo(info)) return info.itemId;
    if (isEditInfo(info)) return info.formId;
}

function isReadOnlyInfo(item: any): item is ReadOnlyInfo {
    return 'itemId' in item;
}

function isEditInfo(item: any): item is EditInfo {
    return 'formId' in item;
}

On this way you get a better ide support

Answer 2

In TypeScript, you can use the getPrototypeOf operator to check the type of an interface object. For example:

interface MyInterface {
  prop1: string;
  prop2: number;
}

const myObject: MyInterface = { prop1: 'hello', prop2: 42 };

console.log(myObject && (Object.getPrototypeOf(myObject) === MyInterface.prototype)) 
// true