maybe as valueOf

maybe.js

let oracle = [];

class Maybe {
  constructor(isNothing, value) {
    this.isNothing = isNothing;
    this.value = value;
  }
  valueOf() {
    oracle.push(this);
  }
}
let Nothing = () => new Maybe(true);
let Just = (v) => new Maybe(false, v);
let stringOfMaybe = (m) => (m.isNothing ? "Nothing" : `Just ${m.value}`);

class BindFunction {
  constructor(f) {
    this.f = f;
  }
  valueOf() {
    let m = oracle.pop();
    oracle.push(m.isNothing ? Nothing() : this.f(m.value));
  }
}

let double = new BindFunction((a) => Just(a * 2));
let m = Just(5);
let n = Nothing();

console.log("m:", stringOfMaybe(m));
m >>= double;
console.log("m >>= double:", stringOfMaybe(oracle.pop()));

console.log("n:", stringOfMaybe(n));
n >>= double;
console.log("n >>= double:", stringOfMaybe(oracle.pop()));