TL;DR I got lucky, and now I'm having challenges getting another position.

I started by working on some simple CMS sites for friends, then taking 3 semesters of basic courses at the local community college with the goal of a career change in my late 30s. I landed my first position solely because a friend worked there and they needed another engineer. There were only 3 of us on the team, very small startup. After two years I started interviewing for another position, and landed something in short order. In retrospect, it was a pretty low-level job - just a PHP hack for a pretty crappy company. Decided to move into a more advanced language and started studying Python. After that job ended I applied for Python jobs through recruiters and landed one with another small company, only 5 of us. It was a good run and I learned quite a bit, but the company closed after running out of money.

I was a bit cocky and thought that finding a new job would be a snap since I had 4 years of experience, and didn't realize how big my gaps in knowledge were. I had four years experience and didn't even know what Big O notation and time complexity is until I was asked about it in my first interview! Crazy but true! I had written APIs, single-page apps, used various front-end and back-end frameworks, and didn't know the basics. I've had about 30 technical interviews that have led to about 10 onsite interviews. I'm getting much better, after bombing the majority of coding challenges I now have a success rate of about 75% of coding challenges leading to onsite interviews. Realize I was quite lucky to get the jobs I did without knowing essentials. Been interviewing 5 months now, which is quite depressing here in the SF area when solid engineers can get multiple offers in a matter of weeks. All I can do is keep learning and plugging along.