First, [4,5,6][1] evaluates to the number 5
Then, the + operator is being applied to a first argument which is an Array and not a Number, so javascript assumes you're doing a string concatenation, not addition. Your array [1,2] becomes a string, which is "1,2". You can see this yourself with [1,2].toString().
The number 5 is now being appended to a string, so it to gets converted to a string, and appended together to get "1,25".
