There are two kinds of functions in the library. Functions with the capitalized nickname of a bird are fixed arity forms of the combinators, e.g. function Kestrel (x, y), function Starling (x, y, z). Functions with a capitalized abbreviated name are curried forms that match the semantics of combinatory logic, e.g. C is the curried form of Cardinal, T is the curried form of Thrush.
The library includes a function that applies normal-order reduction to expressions composed of proper combinators and unknowns. You can type things like reduce('Dxyzw') or reduce('BBxyzw') and get the response 'xy(zw)' to both, thus demonstrating that D = BB. (Or in keeping with the passeri metaphor, "If you call out the name of the bluebird to itself, it responds with the name of the dove.")
npm install oscin.es
Then read readme.md.
To Mock a Mockingbird established the metaphor of songbirds for the combinators, and ever since then logicians have called the K combinator a "kestrel," the B combinator a "bluebird," and so forth. The library's specs contain a comprehensive listing of birds.
In Raymond Smullyan's book, when you call out the song of a bird, each songbird responds with a song denoting a songbird. For example, the Identity bird sings back the name of the bird you sang to it. From the presence of a few simple birds in a forest, you can deduce all of programming.