In classical OpenGL application, after we finish to render a frame, we are usually calling a non-gl method to "flip" the window buffer to our current frame.
But i don't see anything about that in WebGL. How does it work?
WebGL is always back buffered, and the browser pushes the current contents of the back buffer to the front buffer whenever you exit back into the event loop.
So you can perform as many draw commands as you want without fear of showing partial results to the user. When you exit the event loop WebGL will present your changes and clear then backbuffer.
If you want you can also act as if you have a single buffer, performing partial updates as and when appropriate actions prompt them by passing preserveDrawingBuffer: true when creating the WebGL context as in