Can somene help me get the data from a form field and modify it based on its value with javascript? Here is what I have so far, but I dont know how to get the form data into javascript

openerp.sale_custom = function(instance){

instance.sale_custom.SaleClass = instance.web.Class.extend({
    field_val: function(){  /*  pass data from the form into here?... */ },
});

var object = new instance.sale_custom.SaleClass();
object.field_val();

}