What's the difference between writing myLibrary.foobar and myLibrary_foobar? Everyone got too caught up with Java-style module naming convention to bother to fi

What's the difference between writing myLibrary.foobar and myLibrary_foobar? Everyone got too caught up with Java-style module naming convention to bother to figure out how JavaScript actually worked (or to notice that the Java compiler flattens the package hierarchy).