A simple js emitter

emitter.js

const emitter = () => {
  const listener = {};
  const x = {};
  
  x.on = (name, fn) => {
    if (typeof fn !== 'function' || typeof name !== 'string') {
      throw new Error('Wrong parameter(s)');
    }
    
    listener[name] = fn;
    
    return (fn) => { 
      delete listener[name];
    };
  };
  
  x.emit = (name, ...args) => {
    if (typeof name !== 'string') {
      throw new Error('Name must be a string');
    }
    
    if (typeof listener[name] === 'function') {
          listener[name](...args);
    } else {
      throw new Error('Function not registered');
    }
  };
  
  x.off = (name) => {
    delete listener[name];
  };
  
  return x;
};