The running app can be seen here: https://slyg.github.io/blocky/
You will need docker installed on your system.
Just run the make command to build the artefacts, located in the /build folder. It'll create a slyg/blocky-game image as well that you can run using the make serve command.
Typing make help will give you the following options:
make alias for make build
make build builds projects as a docker image (slyg/blocky-game)
make test runs unit tests
make serve runs a slyg/blocky-game container exposing port 8080 (http://localhost:8080)
I thought that starting by using elm, a statically typed and purely functional language, could help me reason about the algorithms I could use, however I haven't have had enough time to write it back in javascript :-/.
I used a recursive breadth first search (BFS) to find the siblings similar colors on the board (named kins in the code), though I'm not entirely sure the algo as is is implemented correctly. It might need more tests cases. See here.
I actually use recursive functions several times, assuming that the board is not too complex and that a stack overflow risk is negligible. A JS runtime might not be the best for such approach I admit.