Event emitter in 11 lines of code

hoki.js

const events = {};

function on(event, callback) {
    if (!events[event]) {
        events[event] = [];
    }

    events[event].push(callback);
}

const trigger = (event, data = null) => events[event].forEach(callback => callback(data));

// Usage:
on('cat-names', console.log);

trigger('cat-names', 'furguson');
trigger('cat-names', 'mittens');
trigger('cat-names', 'boots');