Appcelerator Titanium - Appcelerator Open Source
CodePen - A front end web development playground.
React Native - A framework for building native apps with React
JSFiddle - Test your JavaScript, CSS, HTML or CoffeeScript online with JSFiddle code editor.
Onsen UI - HTML5 Hybrid Mobile App UI Framework - work with Angular, React, Vue, Meteor & pure JavaScript. Material & Flat design.
JS Bin - Sample of the bin: