How to test a decorater usage in typescript?

Question

Suppose there is a property decorator and a class uses that decorator on its certain properties.

function foo(options?: any) {
    return function (target: any, prop: string) {
        // some magic
    }
}

class Bar {
    @foo({ opt1: true }) zoo = 123
}

Suppose that I have covered foo's logic in my unit tests and now am willing to write a test ensures that

Bar class has used foo decorator on its property zoo with option { opt1: true }

How this test should be written ?

P.S. I'm using jest along with ts-jest and open to any other test frameworks if neccessary.

Answer 1

This is an interesting question.

Decorators are meant to be used directly thus cannot be mocked by normal means. This might be a legit case to use jest bypass mock.

However, IMO, the objective of your test is misplaced. You should not test the fact that it has been called with { opt1: true }, you should test the behavior of using such decorator, if possible.

Testing it has been called with { opt1: true } is the same as you want to test your findLCD(a: number, b: number) to make sure it has called Math.abs(a).

You should focus on the behavior (that findLCD(a, b) give you the correct result), instead of how the code does it.

The means, if your decorator @foo does something that is measurable, test against that instead.

For example, if @foo write some log entry, find a way to test the log entry is written, instead of @foo has been called in certain way.