Force calling .toString() on an object when implicitly converted to a string

Question

I'm looking for a way to automatically have an object's toString() method used in cases where it is implicitly converted to a string. For example, say you have this class:

class Dog {
  name: string;
  constructor(name: string) {
    this.name = name;
  }
  public toString(): string {
    return `${this.name} is my friend`;
  }
}

Then I would find that the second of the two assertions in this test will fail:

test.only("Dog", () => {
  const dog = new Dog("buddy");
  expect(dog.toString()).toBe("buddy is my friend");
  expect(dog as any as string).toBe("buddy is my friend"); // fails
});

I get the following error:

expect(received).toBe(expected) // Object.is equality
Expected: "buddy is my friend"
Received: {"name": "buddy"}

(Note: the assertion also fails if I use .toEqual rather than .toBe.)

I would like to make it so that this assertion passes, i.e. when I convert a Dog to a string by some implicitly method like this (as might happen in a TypeScript project that started its life as a JavaScript project).

Any suggestions on if this is possible? Is there some way to modify the Dog class to allow this?

Answer 1

expect(dog as any as string).toBe("buddy is my friend");

as string only changes the type of the object in the typescript compiler. It doesn't do any actual type conversions.

If you check the outputted javscript, you will find plain old: expect(dog).toBe("buddy is my friend");

Just using .toString() explicitly may be your best bet