Whatever people's thoughts on the language itself, JavaScript has built itself into a juggernaut in the amount of tooling available that fit into various opinions that developers can choose from. The number of large frameworks (in terms of popularity and usage) is not really found elsewhere. The number of smaller plugins are vast.

It helps that companies like Google and Facebook have invested a significant amount of research power into designing frameworks and tooling around it. Just from there two companies alone, we have tools like React, Angular, Karma, JSX, Jest, and now Flow. Tooling that involves the browser more include Polymer and Traceur (ES6 to ES5 transpiler).

To contrast this, I have been doing development with Cordova the past week & writing Cordova plugins to fill in missing functionality - the plugin ecosystem with Cordova is horrid, and the documentation is often awful. To compound it, Android developers don't seem to believe in documenting their libraries well.

I will take the JS ecosystem any day when confronted with a choice like that.