multi-methods.js

'use strict';

function multi(pred) {
  var methods = Map();

  var fn = function fn() {
    for (var _len = arguments.length, args = Array(_len), _key = 0; _key < _len; _key++) {
      args[_key] = arguments[_key];
    }

    var methodFn = methods.get(pred.apply(undefined, args));
    return methodFn ? methodFn.apply(undefined, args) : null;
  };

  fn.method = function (predKey, methodFn) {
    methods.set(predKey, methodFn);
    return this;
  };

  return fn;
}

var hello = multi(function (data) {
  return data.type;
}).method('user', function (data) {
  return 'Hello, ' + data.name;
}).method('guest', function () {
  return 'Hello, stranger';
});

console.log(hello({
  type: 'user',
  name: 'Andrey'
}));
console.log(hello({
  type: 'guest'
}));