1. A computing device, comprising: a processor configured to execute computer-executable instructions; and memory storing computer-executable components that, when executed by the processor, cause the computing device to perform a method comprising: rendering, via a web browser of the computing device, a spreadsheet hosted in a backend server, the spreadsheet including built-in functions utilized by a network-based calculation service provided by the backend server to calculate cell values based on data included in the spreadsheet; embedding, into a web page, one or more user-defined functions for extending functionality of the spreadsheet beyond the built-in functions utilized by the network-based calculation service, wherein the one or more user-defined functions are written according to a scripting language and are callable from the spreadsheet; sending registration information for the built-in functions included in the spreadsheet and the one or more user-defined functions embedded in the web page to the network-based calculation service at runtime; instructing, via the web browser, the network-based calculation service to begin a calculation operation that utilizes one or more built-in functions included in the spreadsheet and one or more user-defined functions called from the spreadsheet; receiving, by the web browser, semi-calculated results from the network-based calculation service and a list identifying each user-defined function called from the spreadsheet that needs to be calculated by the web browser to complete the calculation operation, the semi-calculated results including cell values for dependencies that can be calculated by the network-based calculation service utilizing only the data and built-in functions included in the spreadsheet; processing, via the web browser, each user-defined function identified in the list; returning, via the web browser, a value of each processed user-defined function to the network-based calculation service; and rendering, via the web browser, complete calculation results of the calculation operation in the spreadsheet, the complete calculation results including cell values for dependencies calculated by the network-based calculation service utilizing the data and built-in functions included in the spreadsheet and the value of each processed user-defined function.