addEventing.js

'use strict';

var mapWithDefault = require('./mapWithDefault');

module.exports = function addEventing(obj) {
  var handlers = mapWithDefault(function() {
    return [];
  });

  obj.trigger = function(evtName) {
    var args = Array.prototype.slice(arguments);
    args.shift();

    var filterNeeded = false;

    handlers.get(evtName).forEach(function(handler) {
      if (handler.remainingCalls > 0) {
        handler.remainingCalls--;
        handler.fn.apply(undefined, args);
        filterNeeded |= handler.remainingCalls <= 0;
      }
    });

    if (filterNeeded) {
      handlers.set(evtName, handlers.get(evtName).filter(function(handler) {
        return handler.remainingCalls > 0;
      }));
    }
  };

  var onImpls = function(isOnce) {
    var overloads = {
      singleHandler: function(evtNames, fn) {
        return overloads.arrayOfEvents(evtNames.split(' '), fn);
      },
      arrayOfEvents: function(evtNames, fn) {
        evtNames.forEach(function(evtName) {
          handlers.get(evtName).push({
            fn: fn,
            remainingCalls: isOnce ? 1 : Infinity
          });
        });

        return obj;
      },
      mapOfHandlers: function(handlerMap) {
        for (var evtNames in handlerMap) {
          overloads.singleHandler(evtNames, handlerMap[evtNames]);
        }

        return obj;
      }
    };

    return function(arg1) {
      return overloads[
        Array.isArray(arg1)      ? 'arrayOfEvents' :
        typeof arg1 === 'object' ? 'mapOfHandlers' :
                                   'singleHandler'
      ].apply(undefined, arguments);
    };
  };

  obj.on = onImpls(false);
  obj.once = onImpls(true);

  obj.off = function(evtNames, handlerToRemove) {
    evtNames.split(' ').forEach(function(evtName) {
      handlers.set(evtName, handlers.get(evtName).filter(function(handler) {
        return handler.fn !== handlerToRemove;
      }));
    });
  };

  return obj;
};

demo.js

'use strict';

var addEventing = require('./addEventing');

var x = addEventing({});

var fooHandler = function() {
  console.log('foo event for x', arguments);
};

x.on('foo', fooHandler);

x.on('bar', function() {
  console.log('bar event for x', arguments);
});

x.on('foo bar', function() {
  console.log('foo or bar event for x', arguments);
});

console.log('A');

x.trigger('foo');

console.log('B');

x.off('foo', fooHandler);

console.log('C');

x.trigger('foo');

console.log('D');

x.trigger('bar');

console.log('E');

mapWithDefault.js

'use strict';

module.exports = function mapWithDefault(defaultGenerator) {
  if (typeof defaultGenerator !== 'function') {
    var defaultValue = defaultGenerator;

    defaultGenerator = function() {
      return defaultValue;
    };
  }

  var map = {};

  var underlyingMap = {};

  map.get = function(key) {
    var value = underlyingMap[key];

    if (value === undefined) { // We assume that defaultGenerator never returns undefined
      value = defaultGenerator();
      underlyingMap[key] = value;
    }

    return value;
  };

  map.set = function(key, value) {
    underlyingMap[key] = value;
  };

  return map;
};