mgraph.events/index.js

Modern eventing library for Node.js and browsers

// index.js
export default function eventify(subject) {
  if (!subject) {
    throw new Error('Eventify cannot use a falsy object as events subject');
  }
  for (const prop of ['on', 'off', 'fire']) {
    if (Object.prototype.hasOwnProperty.call(subject, prop)) {
      throw new Error(`Subject already has property '${prop}'`);
    }
  }

  // Use a Map to store event listeners:
  const events = new Map();

  subject.on = function (eventName, callback, ctx) {
    if (typeof callback !== 'function') {
      throw new Error('Callback is expected to be a function');
    }
    if (!events.has(eventName)) {
      events.set(eventName, []);
    }
    events.get(eventName).push({ callback, ctx });
    return subject;
  };

  subject.off = function (eventName, callback) {
    // Remove all events if eventName is undefined:
    if (eventName === undefined) {
      events.clear();
      return subject;
    }

    if (events.has(eventName)) {
      // Remove all handlers for this event if callback is not a function:
      if (typeof callback !== 'function') {
        events.delete(eventName);
      } else {
        const filtered = events.get(eventName).filter(
          handler => handler.callback !== callback
        );
        if (filtered.length) {
          events.set(eventName, filtered);
        } else {
          events.delete(eventName);
        }
      }
    }
    return subject;
  };

  subject.fire = function (eventName, ...args) {
    const handlers = events.get(eventName);
    if (handlers) {
      for (const { callback, ctx } of handlers) {
        callback.apply(ctx, args);
      }
    }
    return subject;
  };

  return subject;
}