Add the following function[sourcecode language="javascript"]
function getParameterByName(name) {
name = name.replace(/[\[]/, "\\[").replace(/[\]]/, "\\]");
var regex = new RegExp("[\\?&]" + name + "=([^&#]*)"),
results = regex.exec(location.search);
return results === null ? "" : decodeURIComponent(results[1].replace(/\+/g, " "));
}[/sourcecode]
Then you can use this to get ClientContext[sourcecode language="javascript"]
siteUrl = getParameterByName("SPAppWebUrl");var clientContext = new SP.ClientContext(siteUrl);
[/sourcecode]