How to verify for a specific type using Chai expect() in Typescript?

Question

I've tried looking for an answer in previous posts, but I havnt found one so here it is:

I am currently trying to test the returned type of an Object instance property value using expect() from Chai in Typescript. So far, I've tried this:

/* My type file */

export type customType = "one" | "two" | "three";
export default customType;

/* My test file */

import customType from '{path_to_customType}'; // 'customType' is declared but its value is never read.

const newObject = new Object({property1: value1}) //note: value1 is of type customType

describe("Class1 test", () => {
    it('my tests', () => {
        expect(newObject).to.have.property("property1").to.equal("value1"); //so far, this one works fine
        expect(newObject).to.be.a('customType'); // not working
        expect(newObject).to.be.an('customType'); // not working
        expect(newObject).to.be.a(customType); // not working
        expect(newObject).to.be.an(customType); // not working
        expect(typeof newObject.getProperty1()).to.equal('customType'); // not working
        expect(newObject).to.be.an.instanceof(customType); // not working
        expect(newObject).to.be.an.instanceof('customType'); // not working
        assert.typeOf(newObject.getProperty1(), 'customType'); // not working
        assert.typeOf(newObject.getProperty1(), customType); // not working
    });
});

As you can see, value1 is supposed to be of type customType but it gives me an error since customType is never read. How can I verify if my value is of a specific custom type?

**Note: in my Object definition, the property property1 is specified of type customType

*UPDATE I just noticed the returned type of value1 is 'string' as if the type is only an alias... How can I test that then ?