gistfile1.js

Function.prototype.intercept = function(callback) { 
  var underlyingObservable = this; 
  return ko.dependentObservable({ 
    read: underlyingObservable, 
    write: function(value) { callback.call(underlyingObservable, value) } 
  }); 
}; 

viewModel.lastName = ko.observable("some initial value").intercept(function(value) { 
    // Only accepts strings of length < 20 
    if (value.length < 20) 
        this(value); 
})