Disclosed techniques relate to tracing a subsidiary thread of a web page application. In an example, a method includes providing a web page application to a web browser. The method further includes creating, from the web page application, a global tracer configured to log tracing data for the web page application and a span. The method further includes instantiating, from the web page application, a wrapper for a subsidiary thread. The wrapper is configured to execute shim code before executing the subsidiary thread. The method further includes passing, from the wrapper to the shim code, configuration data of the global tracer. The method further includes creating a subsidiary tracer from the shim code and the configuration data. The subsidiary tracer configured to log tracing data for the subsidiary thread and a thread span.