MACROMEDIA FLASH MX 2004-ACTIONSCRIPT LANGUAGE Reference page 47

v = v + 4 , z++, v + 6;
trace(v); // output: 4
trace(z); // output: 1
The following example is identical to the previous example except for the addition of the
parentheses () operator and illustrates once again that, when used with the parentheses () operator,
the comma (,) operator returns the value of the last expression in the series:
var v:Number = 0;
var z:Number = 0;
v = (v + 4, z++, v + 6);
trace(v); // output: 6
trace(z); // output: 1
See also
() (parentheses)
, (comma) 47