Asynchronous loading of source dependencies is disclosed. An indication is received in the course of executing a script that a source file on which a current class or function depends is not available, wherein the current class or function is called or executed during the execution of the script. Execution of remaining portions of the script, if any, that do not depend on the source file, continues, or other processing tasks are performed if no such portions remain, while the source file is loaded asynchronously.