Feature: assertInstanceOf

[RobDWaller]

New test assertion to allow a developer to assert they have the correct object instance. Useful if there is a polymorphic relationship of some kind.

For example:

class Person {
    age: number;

    constructor(age: number) {
        this.age = age;
    }
}

class Adult extends Person {}

class Child extends Person {}

function definePerson(age: number): Person {
    if (age >= 18) {
        return new Adult(age);
    }

    return new Child(age);
}

console.log(definePerson(17) instanceof Child);
// Output: true
console.log(definePerson(18) instanceof Adult);
// Output: true

[CLAassistant]

All committers have signed the CLA.

[ry]

It this more useful than assert(a instanceof B) ? It's not even shorter to type:

> "assertInstanceOf(a, B)".length
22
> "assert(a instanceof B)".length
22

[RobDWaller]

I appreciate the feedback, it's a fair point, and we could make it assertInstance(). I would argue the interface has value because it is more explicit which makes things easier and more obvious for developers, particularly those with less experience writing tests. It makes it clear you can do this sort of assertion, the assert() method alone does not.

[ry]

@RobDWaller Thanks for the patch but I'm going to close without merge. I think we shouldn't expand the asserts too much.

[RobDWaller]

Thanks @ry understand your position.

I do feel some developers will get benefit from additional assertions though, so will have a look into creating a third party extension they can use if they desire.