The only game demo I can currently find for UE4 in the browser is Flappy Chicken (I know they had some up earlier, but they seem to be taken down). Not really a compelling AAA demo. And their 3D games still only run about half as fast as a native C++ engine. That's very impressive, but you won't be putting out AAA games with a 50% performance hit.
And even then, unreal engine in the browser doesn't run Javascript written by developers. It compiles the C++ code into asm.js. Huge difference. Unless you can code asm.js by hand, you won't be able to do that. Here's a sample of the Flappy Chicken UE4Game-HTML5-Shipping.js file, because the code is just too long and ugly to paste.[0] Definitely not 'JS for gamedev.'
And even then, unreal engine in the browser doesn't run Javascript written by developers. It compiles the C++ code into asm.js. Huge difference. Unless you can code asm.js by hand, you won't be able to do that. Here's a sample of the Flappy Chicken UE4Game-HTML5-Shipping.js file, because the code is just too long and ugly to paste.[0] Definitely not 'JS for gamedev.'
[0] http://i.imgur.com/O3w61XI.png