Try it live https://garrows.github.io/js13k-terrain/
This demo generates some terrain using the midpoint displacement algorithm. The implementation was borrowed from http://www.playfuljs.com/realistic-terrain-in-130-lines/ and modified to be a top down view. Mostly it was done in preparation for http://js13kgames.com.
It also uses https://github.com/dataarts/dat.gui for tweaking.