My question exactly until I read Chroo79's comment. I initially made a whoel if, if else, else statement to factor in null but I love how short and sweet this is.

Hello dani1995ar,

Yes I agree --> keep coding... :)

would't it return true if the length of x = 4 and the length of o = 2? as I understand it

No. Read this: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_AND

The solutions are simple ... but don't loop everything

Some people are better at math than others. It's just the way of things. You could have used a loop instead. Not so clever but just as efficient

Yes, you can visualize it like this:

10                   // no pair
10, 5                // no pair adding to 10
10, 5, 2             // no pair adding to 10
10, 5, 2, 3          // no pair adding to 10
10, 5, 2, 3, 7       // a pair adding to 10: 3 + 7, done