(shameless but relevant plug below)
Caja is reworking its ES5 support via ES5/3 and will do the same for ES harmony. So our structure would be implemented as a Harmony to ES3 layer which can be skipped for real harmony implementations, and then a separable loader that preserves the security properties that concern caja.
Like Traceur, members of the Caja team are part of TC39 (the committee defining ES Harmony).
I don't know about Coffeescript's plans, but it was mentioned during discussions of Harmony modules. Module loaders will likely have the ability to intercept loaded source code (via eval hooks) and rewrite it before module initialization, so if a module is written in CoffeeScript, a runtime CoffeeScript rewriter could be invoked at initialization time. This would allow apps to be composed of modules written in multiple languages that compile down to Harmony at load time.
One thing to note is that not everything in Harmony can be implemented easily via translation. For example, implementing weak maps correctly would require implementing your own garbage collector in JavaScript and even if you did that you would probably just reintroduce the host object/native object cycle problem.
Check out TypeScript, Microsoft's new language based on ES6.
» npm install compile-es6
You can add compiler flags to specify a target, e.g. --target es6. See this question: How do I transpile TypeScript to ES6?
In my case, after adding tsconfig.js and set
{
"compilerOptions": {
"target": "es2015"
}
}
still not working.
Then I run npx tsc instead of npx tsc xxx.ts, then I get 'const'