Semantic differences between original ES6 and transpiled ES3

Referenced (inside closure) before declaration

defs.js detects the vast majority of cases where a variable is referenced prior to its declaration. The one case it cannot detect is the following:

function printx() { console.log(x); }
printx(); // illegal
let x = 1;
printx(); // legal

The first call to printx is not legal because x hasn't been initialized at that point of time, which is impossible to catch reliably with statical analysis. v8 --harmony will detect and error on this via run-time checking. defs.js will happily transpile this example (let => var and that's it), and the transpiled code will print undefined on the first call to printx. This difference should be a very minor problem in practice.