`rollup_bundle` loses sourcemap context due to running tsc for downleveling

[mhevery]

Here is my understand of how rollup_bundle works:

1. copy files into bundles.es6 directory
2. Rollup the files in bundles.es6 directory into bundle.es6.js file
3. Downlevel the file from ES6 -> ES5 into bundle.js using tsc
4. Run uglify to generate bundle.min.js

The pipeline above works fine for the source code, but it brakes source map. There are 2 issues:

1. Many of the actions do not declare *.js.map as output of the action. This can be easily fixed as shown here
2. The source map pipeline is broken. The reason for this is that when tsc is used to downlevel the code from ES2015->ES5 tsc assumes that bundle.es6.js is the source file. This is incorrect assumption. Instead tsc should transform the source maps in bundel.es6.js to the next step. I am not even sure if tsc supports such mode. In any case the issue is that at this point the source map have been destroyed and so the subsequent source maps are of no value.

[alexeagle]

Right, typescript is a broken downleveler for generated inputs.
The ng_package rule solves this by re-building the dependencies with typescript to esm5.

[mhevery]

After discussion with @IgorMinar we feel that the correct path should be:

1. Using tsc generate bundle.esm5 directory with files
2. Rollup bundle.esm5 into bundle.js
3. Run uglify to convert bundle.js into bundle.js.min

[alexeagle]

I fixed this for ng_rollup_bundle by using the hacky esm5.bzl we have in the Angular repo to get a downleveled ESM file straight from TypeScript. But this is a bunch of extra complexity.

One option is to make downleveled-ESM an official third output flavor from ts_library but @mprobst has rejected this idea in the past because of increased complexity in the TypeScript rules, which we don't need internally at Google (because closure compiler does the downleveling itself)

Another option is to push on microsoft/TypeScript#13944 so that TypeScript can be used to downlevel generated JS files.

Another option is to find some other toolchain that does all these steps and handles sourcemaps correctly.

I haven't heard this requested from anyone so I think this issue is low priority.

[connorjclark]

babel accepts input source maps. Any reason tsc must be used for downleveling?

[mprobst]

@alexeagle I'm wary of the combinatorial explosion of emit modes, both for reasons of complexity but also bazel memory usage etc. I think we ought to be able to make do with two modes (development mode and production mode). As Connor says, maybe this is more of a problem with our final downlevelling/bundling tool? microsoft/TypeScript#13944 tracks tsc support for input source maps.