How to check if a parameter passed is the type of my interface

Question

I have a function that takes an object now this object's shape can be of one of three interfaces:

interface A{
strig:string
}
interface B{
number:number
}
interface C{
boolean:boolean
}

The function takes this object an does different things depending on the shape of the object I want to do something like this

function doSomething(item: object){
if(item typeof A) do A effecst;
if(item typeof B) do B effect;
if(item typeof B) do C effect;
}

But I get the error "'IComic' only refers to a type, but here it is used as a value."

Answer 1

Let's try this out for only A first. We have the interface here:

interface A {
    string: string;
}

and now to check it, we have to use something similar to typeof value.string === "string" . Let's return that from a function:

function isA(value: any) {
    // check if value is truthy first
    return value && typeof value.string === "string";
}

Then we just use a type predicate :

function isA(value: any): value is A {

You'll now be able to use it to narrow a value in an if statement (or any condition):

if (isA(aOrBOrC)) {
    aOrBOrC // is now type A
}