minimal POC for Reader monad in JS

1.js

var r = Reader();

console.log(
  r
  .map(env => ({ ...env, y: 2 }))
  .chain(env => Reader(() => ({ ...env, z: 3 })))
  .evaluate({ x: 1 })
);
// { x: 1, y: 2, z: 3 }

2.js

function Reader(evaluate = v => v) {
  return { map, chain, evaluate };
  
  function map(mapFn) {
    return chain(env => Reader(() => mapFn(env)));
  }
  
  function chain(chainFn) {
    return Reader(env => {
      var env2 = evaluate(env);
      return chainFn(env2).evaluate(env2);
    });
  }
}