Depends on the employer and where you live... you may have to get permission or specific exclusion when you start a job. As long as there isn't overlap with the business in question, I haven't experienced an issue working on the side.

Note: working on the side is not drawing multiple FTE salaries... but actual work outside business hours.

I don’t know, the conflict of interest especially in saas is tricky. Let’s assume that I build some niche etl tool, my employer deals with data etl for a certain market, how can i safely determine that my side project doesn’t interfere with their business? Not even lawyers can determine this. And what about the working hours overlap? Who knows except my commits to my repo and myself? I don’t know, side projects seem like a bad advice as they probably increase current job insecurity imo..

You claim it as IP that is previously developed when you start the new job or you submit it to your current employer and get an IP exclusion.

I’ve done both with no issues.

I specifically called out "outside business hours"... there should be no overlap. I also don't tend to work on anything near what I work on at my day job. Same for side work to main job.

Again, it depends on the work environment. I haven't had trouble getting permission when I've wanted/needed it. It's generally if your job performance is suffering, or if you are working in an adjacent or competing market.

You've specifically mentioned two things I believe I excluded in my statement.

The answer is, it depends. Different states have different laws etc...

Also check what your contract says. About non-compete clauses and exclusivity clauses.

If it’s green lights everywhere I will advise you to have another GitHub account than your professional one. And makes everything as separate as possible.

unless you have registered your invention with the employer during the hiring process, it belongs to them

Not true. If your project does not compete with your employer, you do not use their resources, and you do not build it on their time, it is yours. This it the case in California. That doesn't work well at a FAANG because they have their hand in so many pots it is easy to compete with them. Anywhere else, just work on something not related to your work. I'm in security and build in AI.

Incorrect. Without going into all the nitty gritty, even a lot of FAANG companies have explicit moonlighting policies that are fairly permissible.

At the one I am at now, you can get a written sign off from legal before you even start your side project, where they agree that all rights to the side project belong to you. You have to apply for each project individually, but the form is online, just a few fields, and is commonly approved (as long as it isnt a legitimate conflict of interest). I don't mind the process, because each time it nets me an official signed paper that explicitly states that my employer has no rights to my side project.

This is only situationally true, both in terms of employment contract and jurisdiction.

Rule #1: don't listen to sweeping statements about law on HN.

Here [1] is a story that shows a possible scenario of what might happen if your boss fears your side/open source project and believes it directly competes with his company's main mission. Fortunately, the story ends in favor of the engineer.

1. https://news.ycombinator.com/item?id=27424195