Typescript: How to check interface type-CodePudding

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!

CodePudding user response：

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