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 remove the spaces, and it would still work.

a+-+-+-+-+-+-+-+b 

Then I tried

a + + b 

It ran and evaluated to 3, but when I removed the spaces, (a++b) it wouldn't run, and it had a warning which read "Confusing plusses."

I can understand that in cases like

a+++++b 

which could be interpreted as any of the following

(a++) + (++b) (a++) + +(+b) a + +(+(++b)) a + +(+(+(+b))) 

that it would be confusing.

But in the case of

a++b 

the only valid way to interpret this, as far as I can tell, is

a + +b 

Why doesn't a++b work?

Answer 1

The Javascript parser is greedy (it matches the longest valid operator each time), so it gets the ++ operator from a++b, making:

(a++) b 

which is invalid. When you put in spaces a + + b, the parser interprets it like this:

(a) + (+b) 

which is valid and works out to be three.

See this Wikipedia article on Maximal munch for more details.