What is it
JesCov is a Java project and at the moment requires you to run your code using Rhino. If you use JesCov to figure out how well your unit tests cover your code, that should work well for many circumstances.
How does it work
When running code, JesCov will use the Rhino debugger interface to hook into the loading of source files. It will then transform the original source file using an Antlr grammer that inserts coverage-collecting statements. This grammer is based on one that the js-test-driver uses, but heavily modified to accomodate collection of branch coverage as well as line coverage. After modifying the source code, everything will run as usual. At the end of the run, the data collected will be dumped to disk in a JSON-format. This format can then be converted into either Cobertura style XML or a HTML report which annotates source files with coverage information. This functionality exists in the core project, but currently the only way of accessing it is through an Ant-task.