Which Logic Operator Takes Precedence

Question

So, I'm looking into writing a slightly more complex operation with logic operators in an if-else statement. I know I can do parentheses, and I know it's the better way of doing this, but I've gotten curious and so I'm going to ask. If I were to do something like this:

if (firstRun == true || selectedCategory != undefined && selectedState != undefined) {
//Do something
} else {
//Do something else
}

How will that be operated without the use of parentheses? I know there is an order of operations for logic operators, similar to PEMDAS, right? I'm curious if it'll be ran something like this:

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

or maybe if the 'OR' operator takes precedence instead and it ends up going like:

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

The full list would be nice, if you can find it somewhere, of the order of operations for this. Thanks!

Answer 1

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 readability's sake anyway!

Answer 2

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.