This is a project exercise from Free Code Camp's Front-End Developer Certification but I've kind of decided to take the UX a few notches above by adding keyboard inputs, using touch screen inputs when available in order to make the mobile experience more seemless, and making the entire equation viewable in real-time. As a result, I've added back spacing functionality so that mistakes can be addressed without starting over from scratch, the ability to enter the last answer into new equations, and the ability to view the equation in its entirety as well as the result after it has been evaluated. I take some of my functionality cues from the built-in calculator in Google search.
I use the math.js library for the evaluating the expressions so that I can avoid using 'eval', and because it's a powerful library that allows for expanding the calculator's math functions into those befitting of a scientific calculator in the future.
In addition to having a playground in which to practice JavaScript, I am also using the MVC design pattern in order to keep the code as clean and maintainable as possible. Feedback is always welcome on how I might attain this end better than what my current implementation suggests.
Project completed. Jan 16/2017