Now, most of the time, you'll be able to do any side job you want as long as it doesn't get in the way of your main job.
Despite this, there are certain companies that forbid their employees from working on side projects. When it comes to legality, the answer may be a regrettable yes.
The vast majority of employment contracts are at-will, meaning that an employer may terminate an employee for any cause other than illegal.
Therefore, if your company prohibits you from having a second job and you pursue one nevertheless, you may be subject to termination.
However, many companies do not prohibit side jobs outright. Instead, they will limit the type of work you can undertake on the side.
When your employer says you can't side hustle for a competitor in your current field, it's not unreasonable.
Your employer may consider working for a competitor a conflict of interest. Your employer could possibly say you're employing abilities it helped you build for another company.
Alternatively, let's assume you work in marketing, but you want to tutor math on the side because you're exceptionally brilliant at it. It is unlikely that your employer will object.