Have a look at how it is done here
From what I can see I'd guess that it is working and just the error message is false. The problem is that + means both + in the mathematical meaning and string concatenation. So if one part of the equation is a string it means string concatenation. Now have a look at your output message:
"Total damage:" + " " + totalDamage + damageThisRound
And go through it from left to right, then you see that it is always string concatenation. So what you can do is increase the priority by adding () around your calculation:
console.log("Total damage:" + " " + (totalDamage + damageThisRound) );
now it is still string concatenation, but this time (totalDamage + damageThisRound) is a block that gets evaluated before being appended to the string.