Javascript and automatic semicolon insertion

Question

test262 test suite has test containing source:

var x=0, y=0;
var z=
x
++
++
y

The annotation says:

Since LineTerminato

Answer 1

That evaluates to:

var x = 0, y = 0;
var z = x ++ ++ y; //MAKES NO SENSE!

Answer 2

This code will become:

var z = x;
++ ++ y;

The ++ ++ y is the root of the problem. Let’s look at why...

++ ++ y gets evaluated as ++(++y). The first step is to evaluate (++y). The ++ operator increments the value referenced by the variable it is next to, and returns the incremented value. The important part here is that it does not return a reference, just a value. So the second step would be ++(1), (or whatever ++y yielded), which is an error, since only references can be incremented.

Answer 3

The grammar does not allow a new-line to precede a ++ or -- operator; such a new-line must be converted to a ;. Consequently, the expression must be parsed as though it had been:

var x = 0 , y = 0 ;
var z = x ;
++ ;
++ y ;

The third line is illegal.

References:

Section 7.9.1, "Rules of Automatic Semicolon Insertion", rule 3

Section 11.3, "11.3 Postfix Expressions".