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dhtmlxform
Events

# onBeforeChange Event

Event occurs before data in some input changed ( by user actions )
Event works for constructed input elements only

Parameters for inputs

- an item id
- an existing value
- a new value

    myform.attachEvent("onBeforeChange", function (id, old_value, new_value){
        //any custom logic here
    return true;
    });

Returning false from event handler will block value changing.