completed the product-invory.js and the unit tests realted to it#18

DeepthiGirijaThampi wants to merge 1 commit intoLaunchCodeEducation:mainLaunchCodeEducation/software-dev-course-unit-testing:mainfrom

DeepthiGirijaThampi:feature-devDeepthiGirijaThampi/software-dev-course-unit-testing:feature-devCopy head branch name to clipboard