multimethod.js

function createMultimethod(dispatch, noMatch) {
  if(typeof dispatch !== 'function') {
    throw new TypeError('dispatch must be a function');
  }

  const dict = {};
  if(typeof noMatch == 'function') {
    dict.noMatch = noMatch;
  }

  return new Proxy(() => { throw new Error('No match'); }, {
    set(target, property, value) {
      dict[property] = value;
      return true;
    },
    apply(target, thisArg, args) {
      const value = dispatch.apply(null, args);
      const func = (value && dict.hasOwnProperty(value)
                    ? dict[value]
                    : (dict.hasOwnProperty('noMatch')
                       ? dict['noMatch']
                       : target));
      return func.apply(thisArg, args);
    }
  });
}

// Usage:
let dispatchFunc = person => person && person.manners;
let noMatchFunc = () => { return 3.0; };
let coffeePrice2 = createMultimethod(dispatchFunc, noMatchFunc);

coffeePrice2.polite = (person) => {
  return 2.00;
};

coffeePrice2.rude = (person) => {
  return 6.00;
};

let alice = { name: 'Alice', manners: 'polite' };
let bob = { name: 'Bob', manners: 'rude' };

console.log('$' + coffeePrice2(alice)); // $2
console.log('$' + coffeePrice2(bob));   // $6

console.log('$' + coffeePrice2()); // $3

coffeePrice2.veryPolite = (person) => {
  return 1.85;
};

let carol = { name: 'Carol', manners: 'veryPolite' };
console.log('$' + coffeePrice2(carol)); // $1.85