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Sandboxing
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To create the sandbox, Facebook removed many of the offending functions and access to the global objects that might be misusedWhen an FBML application canvas page loads, or profile content is set via the profilesetFBML() Facebook API method, Facebook scans it for JavaScript If it finds any, it automatically places it in a special namespace created by prepending every JavaScript function, object, array, variable, and event reference it finds with an identifier that contains the current application s IDThis action ensures that there are no clashes with variables defined in the global namespace and effectively removes the ability for this JavaScript to obtain any reference to global objects or functions Facebook also hides the powerful JavaScript eval() function and any global functions that might be used as attack vectors Finally, Facebook significantly modifies the DOM object modelAccess to the global Window object, which is the root object and default execution context in all client-side JavaScript programming, is forbiddenAlso gone is the darling of most client-side web programmers, the DOM-provided alert() function DOM properties themselves are also restricted and only a subset is exposed; however, instead of properties, they are replaced with get and set functions Event handlers are also significantly modified Many common event handlers familiar to JavaScript programmers, such as the onload() functions, are not available, and events themselves are also modifiedThese are just some of the changes that Facebook made to JavaScript to create its safer, sanitized Facebook JavaScript (FBJS) One of the first questions new Facebook developers have about FBJS is the availability of third-party JavaScript library supportWith the explosion of AJAX-enabled sites, many libraries Dojo, Mochikit,YUI, Prototype, Scriptaculous, and jQuery, to name a few have been developed that provide loads of extra functionality, objects, and primitives to JavaScript developersThe short answer is that none of these libraries are available in FBJS If you need them for your code, you must use an IFrame-based canvas page to do so
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Sandbox Creation and Initialization
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To get a better understanding of the sandbox, we present a simple example using the FBML Test ConsoleTo use it, log in to Facebook and visit the Developer Tools page at http://developerfacebookcom/toolsphp Click the FBML Test Console tabType this code into the FBML text field within the console:
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<a href="#" id="hello">Hello World!</a> <script> documentgetElementById('hello'); </script>
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Now, select Canvas from the Position drop-down menu Next, click the Preview button and examine the HTML Source window s contentsAs you can see, Facebook creates a lot of JavaScript in the background for the simple documentgetElementId() call made from a canvas page Listing 111 shows this content in detail (with irrelevant features
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11 FBJS, Mock AJAX, and Flash
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removed or commented out), and we bolded the code from the previous small program to make it easier to see in context
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Listing 111 JavaScript Generated for a Simple Hello World FBJS Script
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<a href="#" id="app2353941073_hello" onclick="(new Image())src = '/ajax/ctphp app_id=2353941073&action_type=3& post_form_id=e7270c8ceb77f9ce89258268a463a860&position=3&' + Mathrandom();return true;" fbcontext="0dab11581b51"> Hello World! </a> <script type="text/javascript"> var app_2353941073 = new fbjs_sandbox("2353941073")setBridgeHash(""); app_2353941073validation_vars = { <! 'fb_sig'-style variables > }; app_2353941073context = "0dab11581b51"; app_2353941073contextd = [JSON code not relevant to discussion]; app_2353941073data = { "user": 714497440, "installed": true }; app_2353941073bootstrap(); </script> <script type="text/javascript"> a2353941073_documentgetElementById('hello'); </script> <! lots of JavaScript omitted > </script>
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The most important thing happening in Listing 111 is the creation of the sandbox itself Facebook creates the sandbox instance with the code line shown here:
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var app_2353941073 = new fbjs_sandbox("2353941073")setBridgeHash("");
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Notice that the new sandbox is assigned to a variable called app_[app id], and the application ID is passed as a parameter to the fbjs_sandbox() constructorAs mentioned, Facebook uses this ID to rename all the functions and variables in your FBJS to ensure that they do not conflict with any other code in the global namespace Listing 112 shows the implementation of fbjs_sandbox()
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