The article has good points. But the term "side project" implies working on something that isn't part of your job. I think looking at your project from that

The article has good points. But the term "side project" implies working on something that isn't part of your job. I think looking at your project from that perspective can also be a step in the wrong direction---if you enjoy what you do, work is just one such application of your knowledge and ability. Projects you do outside of work are another, and may even be more important to you than work. A number of commenters point out that many side-projects to them are to explore new concepts/libraries, or as a means of learning. If it's for the sake of learning for your job, then yes, it'd be a side project. But if you're looking to take a project to completion (the author talks about products; my projects aren't), I find that using it as a learning environment extremely detrimental. Yes, you do learn things over the course of development---through research, struggle, and growth. But if too much of your time is spent on things you don't have a good foundation on, you may burn out too early, wind up frustrated, and wind up with a mess that leads to refactoring. Use a foundation that you have experience with and know well, and learn the parts that are necessary. (I do distinguish, though, a research project with a project for playful learning. The former is much more formal and disciplined. But you can still burn out early.)