1. A method of rendering a web page comprising: at a client computer system with one or more processors and memory: identifying, for rendering, a web page selected by a user, wherein the web page references a loadable script that is separate from the web page, and wherein the script includes executable code and a plurality of function definitions; while opening the web page: instantiating a variable; and asynchronously loading the script such that the web page is operable within the client computer system while the script is being loaded; in response to a user interaction with the web page at the client computer system: identifying a function associated with the user interaction, wherein a definition of the function is in the script, and the definition is not currently loaded; inserting into the variable a command string representing the function; and after the plurality of function definitions in the script are loaded: executing the function in the script corresponding to the command string that was inserted into the variable.