Java Script Numbers and string concatenation- Activity

In Java Script Numbers and string concatenation- Activity the question is given as console.log("1"+2+3) for which the output is 123. My question is that why it doesn't add last two number and then concatenate with string "1" as 15. Teaching staff please explain how.

This question arise because the previous question is console.log(1+2+"3") in which the first two numbers are added and then concatenate with the string "3".