I'm sure I'm not the only one who at some point wondered how JS works under the hood, understand closures and types and why typeof null is an object?!?
I recommend this course from udemy by Toni Alicea - Javascript: Understanding the weird parts (I purchased on Cristian's recommendation).
I got it for less than £15, but sometimes it goes up to £60, so just follow it up for when they offer discounts.