The IO monad, in JS

IO.js

class IO {
  constructor(val) {
    this._fn = ()=>val;
    return this;
  }
  static fromFn(fn) {
    const ret = new IO();
    ret._fn = fn;
    return ret;
  }
  map(fn) {
    return IO.fromFn(()=>fn(this.runIO()));
  }
  flatten() {
    return IO.fromFn(()=>this.runIO().runIO());
  }
  runIO() {
    return this._fn();
  }
}