You can apply simple math functions to calculate fields using javascript. For example, if you want to get sum of two or more fields, or get sum of field with multiplication.
In the below example, we are getting sum of three fields and substracting one field on change event. If you don't specify the $eventFields argument then it will automatically apply change event on each of the formula fields so it will calculate on the change on the all these four fields.
$pdocrud = new PDOCrud();
/**
* Set/Call js actions for the form elements on some js event
* @param string $element Main element, On which element, calculated value should be displayed
* @param string $formula Formula to be applied
* @param string $event Javascript event, this will be applied on formula fields if no event fields passed
* @param string $eventFields Event fields if any
* return object Object of class
*/
$pdocrud->setJsActions("order_total", "{order_sub_total} + {tax} + {shipping} - {discount}","change");
echo $pdocrud->dbTable("x_ordertable")->render("insertform");