How to add onchange javascript method for validation purpose

Hi,
I am new to this. I have a jsp page and it populates the data using smart client and now I am trying to add a javascript method to the drop down.

sTRING colDef
colDef += "editorType: 'SelectItem', ";
colDef += "optionDataSource: '<data>', ";
colDef += "displayField:'description', ";
colDef += "valueField:'description', ";
colDef += "pickListWidth:400, ";
colDef += "pickListProperties: {showFilterEditor:true},pickListFields:[{name:'description'}],";

var channelSizeProfileColumns = [
{name:"description", title:"Description"}
]
isc.DataSource.create({
ID:"<ID?",
clientOnly:true,
fields:colDef,
testData:<data>,
changed:test()
})

How do I add the onchange javascript method here?PLease help!