This expression...
"1"- -"1"
... is processed as ...
"1"- (-"1")
... that is, substract result of unary minus operation applied to "1" from "1". Now, both unary and binary minus operations make sense to be applied for Numbers only - so JS converts its operands to Numbers first. So that'll essentially becomes:
Number("1") - (-(Number("1"))
... that'll eventually becomes evaluated to 2, as Number("1")
, as you probably expect, evaluates to 1
.
When trying to understand "1"--"1"
expression, JS parser attempts to consume as many characters as possible. That's why this expression "1"--
is processed first.
But it makes no sense, as auto-increment/decrement operations are not defined for literals. Both ++
and --
(both in postfix and prefix forms) should change the value of some assignable ('left-value') expression - variable name, object property etc.
In this case, however, there's nothing to change: "1" literal is always "1". )
Actually, I got a bit different errors (for x = "1"--"1") both in Firefox:
SyntaxError: invalid decrement operand
... and Chrome Canary:
ReferenceError: Invalid left-hand side expression in postfix operation
And I think these messages actually show the reason of that error quite clearly. )