The OP is not talking about mundane things, such as remember the difference between implode and explode in PHP. They are talking about a a deep feeling of incompetence. I believe that imposter syndrome is more prevalent in the software world because, unlike auto mechanics or surgeons for example, we are not give a chance to watch the process actually take place, we only see the results. We see the finished code and don't know how it was derived. Imagine all we got was: car works again, now you fix the next one

If someone writing a novel has to look up the spelling of a particularly useful word that fits the situation perfectly, but they can never manage to spell it right without aid, does that make them an impostor?

No brain can contain the exact right way to do x, y and z - and it needn't, because we have a magic box that we can ask and get great help when we need it.

IMO, being a developer isn't about being 100% amazing with your syntax or being able to recite the ES2015 spec by heart; it's about knowing what tools to use when.

You are definitely not alone.

You say that you have to keep looking stuff up and watching tutorials. I hear that you're constantly learning and that's never a bad thing. You should be worried when there comes a day when you feel like you know everything because that's definitely not normal.

“Over a decade” is a small time being a professional. You’re what, about 35 on average?

I'm 53. I have a 25 year IT career and the last 10 of it has been coding and sql server mostly.

Thank you for intensifying my impostor syndrome ;)