It is amazing what can now be done with JavaScript ... perhaps one of the most-studied language implementations in the world ... and the case for pure-C/C++ coding is definitely shrinking thereby.   But there is still a gap.   And, many of the “optimizations” that are being done in the JS world effectively consist of the clever application of XS behind-the-scenes.   Tools like Cordova make such things more explicit, but there is still an “external function interface” lurking behind.   Even so, it is often used to “get directly to the hardware” [of a mobile this-or-that].   The number of cases where “the slight overhead of an interpreter can’t be tolerated” is quickly shrinking, for a number of reasons, both hardware and software.