It looks like the requisite debugger technology is just coming online in dev builds for Mozilla: https://hacks.mozilla.org/2013/05/compiling-to-javascript-an...

Chrome has some sort of support for it too, which Closure can apparently use. (That's Closure the JS library, not Clojure the JVM Lisp.)

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.

That's an interesting question. Debuggers are very different when they target functional languages and the mote you express with fewer lines of code make for somewhat surprising.

OTOH, the way one codes in functional languages is also different. Lack of a debugger may not he a problem as big as it would be with, say, Java.

No, its plain old lazyness. All these "compiled to JS" language makers would rather add more fancy syntactic sugar or extend the standard library than getting their goddamn bases covered.

I have no problem with that, but please don't go on about how you are much more productive in this language than in JS, and how much cheaper the code will be to maintain.

I think it's ridiculous to call people "lazy" who wrote an entirely new language from scratch and released it for free, because it's lacking one feature.

Per my sibling post, I do not believe you are technically correct. Debuggers must support the ability to debug compiled languages before anyone can usefully implement support for debugging. I'm curious what exactly you think the developers of languages that compile to JS are not doing out of "lazyness"?

A counterexample is coffeescript, which I think does have a plugin for Chrome to allow a source-level debugger.

You can run (and debug) Dart natively in Dartium.