=======================
on_before_field_changed
=======================

on_before_field_changed(field)

**domain**: client 

**language**: javascript

**class** :doc:`Item class </refs/client/item_api>`

Description
===========

Write an ``on_before_field_changed`` event handler to implement any special 
processing before field’s data has been changed.

The ``field`` parameter is the field whose data is about to be changed. To get the 
item that owns the field, use the 
:doc:`owner </refs/client/field/at_owner>` 
attribute of the field.

Before triggering this event handler the application assigns the new value that is 
about to be set to the ``new_value`` attribute to of the field. You can change 
the value of this attribute. This value will be used to change field’s data. 


Example
=======

.. code-block:: js

    function on_before_field_changed(field) {
        if (field.field_name === 'quantity' && field.new_value < 0) {
            field.new_value = 0;
        }
    }

See also
========

:doc:`Fields </programming/data/fields>`

:doc:`value </refs/client/field/at_value>`

:doc:`on_before_field_changed <on_before_field_changed>`