Which Logic Operator Takes Precedence

My rule of thumb, which covers basically 99% of all use cases for conditional statements, is:

1. Grouping: ()
2. Member access . or [...]
3. Not: !
4. Comparison, e.g. < , >= , === , !=, ...
5. Logical AND &&
6. Logical OR ||

MDN gives you the exhaustive breakdown: Javascript Operator Precedence

so for your example:

(firstRun == true || selectedCategory != undefined && selectedState != undefined)

equals

(firstRun == true) || ((selectedCategory != undefined) && (selectedState != undefined))

For anything more complex than the above mentioned cases I would look into refactoring the code for readabilities sake anyways!

There is a pretty good rule of thumb to this. Think of these operators as of mathematical ones:

• AND is multiplication (eg. 0 * 1 = 0 => FALSE)
• OR is adding (eg. 0 + 1 = 1 => TRUE)

When you remember this, all you have to know is that multiplication always comes before addition.