Question

I am using GWT and the translated javascript has this functionality in several places

valuesLength = (values.length , undefined);

I assumed that it was a set to undefined if object does exist but when debugging in the browser although values object exist and length has a value valuesLength always equals undefined.

Answer 1

Its because the comma operator evaluates its operands left to right and returns the value of rightmost operand.

(values.length , undefined)
               ^^                  // will output undefined

MDN reference (not working right now)

If the objective is to have check for object’s existence, the right way to do that is

var valuesLength;
if(values && values.length) {
  valuesLength = values.length;
}
// else valuesLength will be undefined by default