EventEmitter - functional style

event-emitter.js

// --- EventEmitter Factory Implementation ---

const { call, numberCalls, removeItem, append } = require('./helpers');


const unsibsribe = (_handlers, event, handler) => {
  _handlers[event] = removeItem(_handlers[event], handler);
};

const on = _handlers => (event, handler) => {
  _handlers[event] = append(_handlers[event], handler);
  return () => unsibsribe(_handlers, event, handler);
};

const once = _handlers => (event, handler) => on(_handlers)(event,
  function _handler(...args) {
    unsibsribe(_handlers, event, _handler);
    return handler(...args);
  }
);

const emit = _handlers => (event, ...args) => (
  numberCalls(_handlers[event], call(args)) > 0
);

const createEventEmitter = (handlers = {}) => ({
  on: on(handlers),
  once: once(handlers),
  emit: emit(handlers),
});

module.exports = { createEventEmitter };

helpers.js

// --- HELPERS ---

const call = args => func => func(...args);

const numberCalls = (funcs, mapper) => (
  funcs && funcs.length ? funcs.map(mapper).length : 0
);

const removeItemAtIndex = (arr, index) => (
  index < 0 ? arr : arr.slice(0, index).concat(arr.slice(index + 1))
);

const removeItem = (arr, item) => (
  arr && arr.length ? removeItemAtIndex(arr, arr.indexOf(item)) : arr
);

const append = (arr, item) => (
  arr && arr.concat(item) || [item]
);

module.exports = { call, numberCalls, removeItem, append };

usage-sample.js

// --- createEventEmitter - Usage Sample ---
const { createEventEmitter } = require('./event-emitter');


const eventEmitter = createEventEmitter();

eventEmitter.once('click', (...args) => console.log('First click: ', args));
eventEmitter.on('click', (...args) => console.log('Every click: ', args));

console.log('-----------------');
console.log(
  'Event handled:', eventEmitter.emit('click', 1)
);
// -----------------
// First click:  [ 1 ]
// Every click:  [ 1 ]
// Event handled: true

console.log('-----------------');
console.log(
  'Event handled:', eventEmitter.emit('click', 1, 2)
);
// -----------------
// Every click:  [ 1, 2 ]
// Event handled: true


console.log('-----------------');
console.log(
  'Event handled:', eventEmitter.emit('click', 1, 2, 3)
);
// -----------------
// Every click:  [ 1, 2, 3 ]
// Event handled: true

console.log('-----------------');
console.log(
  'Event handled:', eventEmitter.emit('hello', 'hello')
);
// -----------------
// Event handled: false