After working with something for some time you'll learn/know the important things automatically.
There is no need to know all APIs, function parameter lists, etc. There are docs for things like that. Generally spoken, you need to know things or where to find them.
For example, let's take the $.ajax() function of jQuery. I know what it does and the commonly used options such as url, type, data, etc. If I need something non-standard, I open the docs and check them.
When looking up stuff you need to be sure that your sources are trustworthy and up to date. For example, MDN is while w3schools isn't.