JavaScript is and always will be the language of the Web. It survived and overlived very difficult times of total rejection. It was some years back when JavaScript had seen some love and understanding. Now it's one of the most loved and used languages out there!
JavaScript is still evolving and changing. It's on the right track of being a serious and mature language. It will take more time though. The first major step was ECMAScript 6, which introduced various needed and long-awaited features. ECMAScript 7 is at the corner with even more intriguing features. The ES committee will now release new versions of ECMAScript more frequently, several times per year, meaning JavaScript will come to its sweet spot pretty soon.
So, is Javascript suitable for serious app development? - YES, if used property, and by property I mean used for building web applications.
Almost every company (if not every) use JavaScript nowadays - Google, Microsoft, Apple, Facebook, Quora, Github, Hashnode/Devmag to name few. Even Wordpress, the biggest CMS out there, powering 45% of the web, moved to Node and React.
Speaking of Node, being the best thing that could happen to JavaScript, still has a long way to go, before recognized as a serious Back-End and desktop technology. But it's surely on the right track.
We surely have interesting years ahead of us!