The sum function will get the sum of an array. If you try and sum an array which is not numeric fields, the function will crash.
Example
pjs.define("Num", { type: 'integer', length: 10, dim: 5 });
Num[1] = 1;
Num[2] = 2;
Num[3] = 3;
Num[4] = 4;
Num[5] = 5;
return Num.sum();
//15