Do source maps also support tracing values in variables, or do they only map line numbers in source to line numbers in output? Because the former (mapping values) is the hard part of implementing a debugger.

I don't think tracing values is very important when they're immutable.

How are they immutable? Given some obj, is obj.prop immutable?

Actually that’s Closure the Javascript compiler. There’s nothing special that has to do with source maps in Closure Library, because it’s regular JS.