My barebones.js webGL library now has a basic heightmap-based terrain and simple camera system:

It is hopefully simple enough and isolated enough that you can borrow it, adapt it, or learn from it without needing to adopt barebones.js.

It does top-down and 3D views, and makes it easy to work out where the user clicked and to update the map tiles.  Grid is optional.

Each tile can have its own texture etc.

So go make an RTS or something!

Good luck!