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.

See this chart for precedence.

I'm not going to explain what happens because the next guy reading your code will think: "WTF? Does that do what it should?"

So the better solution is to wrap the terms in parentheses even if you know the precedence, applied it correctly and the code works

This follows the old wisdom that you shouldn't do everything you can just because you can do it. Always keep an eye on the consequences.