add-eventing.js

const addEventing = function (obj) {
  let events = {}

  obj.on = (eventName, fn) => {
    if (events[eventName]) {
      events[eventName].push(fn)
    } else {
      events[eventName] = [fn]
    }
  }

  obj.trigger = (eventName, ...args) => {
    events[eventName].forEach(fn => fn(...args))
  }

  return obj
};

//
//  Make an addEventing mix-in that adds .trigger() and .on() methods to the input object.
//
//  Example:

var obj = addEventing({ name: 'John', age: 35 });

obj.on('ageChange', function() {
  console.log('age changed');
});

obj.trigger('ageChange');
//  --> Calling this logs: 'age changed'
//
//  Requirements:
//  - addEventing should return the original object it was passed.
//  - It should be able to handle multiple callback functions for the same event name.
//  - If obj.trigger is called with additional arguments it should pass those to the listeners.
//  - Note: we don't need to write a way to remove event listeners.
//
//  ----------------------------------------------------
//  Example 1:

var obj2 = addEventing({ name: 'Bob', age: 30 });

obj2.on('ageChange', function() {
  console.log('Age changed');
});

obj2.trigger('ageChange');

//  --> logs: 'Age changed'
//
//  ----------------------------------------------------
//  Example 2:

var obj3 = addEventing({ name: 'Bob', age: 30 });

obj3.on('ageChange', function() {
  console.log('Age changed');
});

obj3.on('ageChange', function() {
  console.log('Age changed again');
});

obj3.trigger('ageChange');

//  --> logs: 'Age changed'
//            'Age changed again'
//
//  ----------------------------------------------------
//  Example 3:

var obj4 = addEventing({ name: 'Bob', age: 30 });

obj4.on('ageChange', function(age) {
  console.log('Age changed to: ' + age);
});

obj4.age++;

obj4.trigger('ageChange', obj4.age);

//  --> logs: 'Age changed to: 31'