Elm-redex is a lambda calculus toy that I wrote in 8 hours in Elm and a little Javascript.
It is available live on my website here.
To build it, currently a bug in Elm means that it must be built against revision 1c6dfdeb or later of the Elm compiler.
Parsimmon, the JS library, is also used for parsing lambda terms.
To build, just invoke the Elm compiler as shown:
$ elm --only-js Main.elm
And then open build/Main.html. Have fun!