Why does “a + + b” work, but “a++b” doesn't?

Question

I was fiddling around with different things, like this

var a = 1, b = 2;
alert(a + - + - + - + - + - + - + - + b); //alerts -1

and I could

Answer 1

Javascript's operator precendence rules for the ++ increment operator has no left-right associativitity. That means a++b can be interpreted as a++ b or a ++b depending on the particular implementation. Either way, it's a syntax error, as you've got 2 variables, one unary-operator, and nothing joining the two variables.

In practical terms:

a = 1
b = 2;

a++ b; -> 2 2
a ++b; -> 1 3

What does 1 3 mean as JS code?