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."

i tried this if(item is A) but i still get the error "'IComic' only refers to a type, but here it is used as a value." IComic is one of my interfaces — ludinj
– ludinj, Commented Oct 11, 2022 at 3:18

k.tten · Accepted Answer · 2022-10-11 03:36:31Z

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 }

the problem i have is im trying to do this inside a reducer swicht stament and each interface has many properties

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