Source maps for bundle mode

[ribrdb]

J2CL recommends using bundle mode for development, but that means we don't get any sourcemaps. Is there any reason the compiler can't generate the .map in bundle mode?

[blickly]

Created internal issue http://b/127860990

[niloc132]

We have a rough patch that makes this possible in our build workflow, created so that https://github.com/google/j2cl/ can cleanly use sourcemaps in the browser debugger. The code in google/j2cl seems to assume that this is actually already possible:

https://github.com/google/j2cl/blob/master/build_defs/internal_do_not_use/j2cl_js_common.bzl#L44-L53

    closure_js_binary(
        name = name,
        compilation_level = "BUNDLE",
        defs = entry_point_defs,
        deps = deps,
        # For J2CL it is in impractical to embed all source into sourcemap since
        # it bloats sourcemaps as well as it slows down bundling.
        nodefs = ["--source_map_include_content"],
        **kwargs
    )

Vertispan@f64876e

Note that this patch doesn't actually bundle up sourcemaps in the JS or process in any way (merging, etc), just points to where those files are on disk, under the assumption that this is the easiest way to serve content file-by-file. Also, due to how Chrome interprets sourceURL directives in a given .js file as being relative to the original .html file, this requires that the .js file end up in the same directory as the .html file (unlike firefox, which works as I would naively expect here).

[johnthuss]

I'd really like to be able to have source maps for development using J2CL, specifically with the _dev_server task.