Introduction to JavaScript source maps
HTML5Rocks has a tutorial introducing JavaScript source maps, currently available in Chrome Canary or WebKit nightly (not Firefox love yet).
For everyone who’s made the jump to Coffeescript, the one knock against the language is that while it writes readable JavaScript, it’s not your code, so debugging is harder.
JavaScript source maps gives the browser the ability to map generated code back to the line of compiled code that created it.