Techniques are described herein that are capable of virtualizing extension code (e.g., third-party extension code) in an application (e.g., a web application in a browser). A portal of an application is loaded. The portal has an extensibility point via which an extension is to enhance functionality of the application. The extension includes metadata and implementation code. A functionality that the implementation code is configured to provide with regard to the application is automatically determined. The metadata is automatically generated to indicate the functionality in response to the functionality being automatically determined. The metadata is loaded in response to the portal being loaded. Loading of the implementation code is delayed until an interaction indicator is received, in response to loading the metadata. The interaction indicator indicates that a user interaction occurs. The implementation code is loaded based at least in part on receipt of the interaction indicator.