An example container used for functional programming, to help separate impure functions from pure logic.

container.js

const isFunction = fn => fn && Object.prototype.toString.call(fn) === '[object Function]';

const isAsync = fn => fn && Object.prototype.toString.call(fn) === '[object AsyncFunction]';

const isPromise = p => p && Object.prototype.toString.call(p) === '[object Promise]';

const tap = f => x => {
  f(x);
  return x;
};

class Container {
  constructor(fn) {
    this.value = fn;
    if (!isFunction(fn) && !isAsync(fn)) {
      throw new TypeError(`Container expects a function, not a ${typeof fn}.`);
    };
  }

  map(fn) {
    if (!isFunction(fn) && !isAsync(fn)) {
      throw new TypeError(`The map method expects a function, not a ${typeof fn}.`);
    };

    return new Container(
      () => isPromise(this.value()) ?
        this.value().then(fn) : fn(this.value())
    )
  }

  run() {
    return this.value();
  }
};

const sayHello = () => 'Hello';
const addName = (name, str) => str + ' ' + name;

const container = new Container(sayHello);
const greet = container
  .map(addName.bind(this, 'Joe Bloggs'))
  .map(tap(console.log));

greet.run();