knockout.js ko.computed function for two textboxes

This example shows you how to work with ko.computed

HTML

<input id="test1" name="test1" type="text" data-bind="value: searchQuantity1, valueUpdate:'afterkeydown'"/>
<input id="test2" name="test2" type="text" data-bind="value: searchQuantity2, valueUpdate:'afterkeydown'"/>

<p>Test1 value is : <span id="spantest1" data-bind="text: searchQuantity1"></span></p>
<p>Test1 value is : <span id="spantest2" data-bind="text: searchQuantity1"></span></p>
<p>Sum: <span id="spantest3" data-bind="text: spans"></span></p>

JavaScript

var viewModel = function(data) {
   var self = this;
   self.searchQuantity1 = ko.observable(1);
   self.searchQuantity2 = ko.observable(2);
   self.spans = ko.computed(function() {
       return 
};

ko.applyBindings(new viewModel());