Note that Window.js does not expose GLFW directly; it exposes APIs that are more similar to the web, for familiarity to web devs.
For example, GLFW has this for mouse input:
https://www.glfw.org/docs/latest/input_guide.html#input_mous...
But Window.js wraps that in window.addEventListener and "mousedown" events:
https://windowjs.org/doc/window#window.addEventListener https://windowjs.org/doc/window#event-mousedown
If you give it a try to build a game then keep in mind that it doesn't have a sound API yet, though that's part of the future plans.
Finally, is there something specific in libsdl that you'd like to have and Window.js doesn't support?