Preserving TypeScript/CoffeeScript Source Map through UglifyJS.

I was pleased to see that UglifyJS can transform the source map from TypeScript through to the Optimized JS:

--in-source-map    Input source map, useful if you're compressing JS that was
                   generated from some other original code.

The motivator was that I am getting ready to compress JS/CSS in my project, but my JS is generated from TypeScript and I wanted to keep the Source Map. I hoped that there would be a way to carry it through the process, so I can debug all the way back to TypeScript. I thought it should be pretty easy to do—in theory just one more transformation—and hoped that a tool existed to merge two source maps (ie. I wouldn’t have to write it)… it exists, and I’m pleased with UglifyJS!