Discussion on: Pro tip: using Promise.then for function composition

The cool thing about Promises is that the then method, when used this way, is just like the map function. It has the same abstract type signature. This makes Promises behave like any other functor, which means that this method respects composition. Therefore,

somePromise
  .then(f)
  .then(g)
  .then(h);

is the same thing as

const w = pipe(f, g, h);
somePromise.then(w);