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In Facebook, user pages are updated with the last messages from friends without the need to reload the page. To achieve this, Facebook uses Ajax: a set of Web development techniques for asynchronous client-to-server communication. However, when page elements (e.g. the last messages) depend on Ajax responses, they must be updated after every communication with the server to show up-to-date information. The complexity of Ajax updates can be alleviated by introducing basic support for reactive values: when a value originating from an Ajax response changes, all page elements depending on it are updated accordingly. Using AspectScript, we introduce a library for basic reactive values.

The implementation follows: