The IO monad implemented with JavaScript factory function

IO.js

function IO(effectFn) {
  const __val = effectFn;
  const map = fn => IO(() => fn(__val()));
  const performUnsafeIO = () => __val();
  const chain = fn => IO(() => fn(__val()).performUnsafeIO());
  return Object.freeze({
    map,
    chain,
    performUnsafeIO
  });
}