JavaScriptで1と2と3を足す

>>> 1 + 2 + 3
6
>>> "1" + 2 + 3
"123"
>>> 1 + "2" + 3
"123"
>>> +"1" + 2 + 3
6
>>> +"1" + 2 + "3"
"33"
>>> + + 1 + + "2" + + 3
6
>>> ++ 1 ++ 2 ++ 3
ERROR
Uncaught ReferenceError: Invalid left-hand side expression in postfix operation