Count to a certain number

Counter.js

const Counter = {
  handler: null,
  end: 5,
  seconds: 0,
  start: () => {
    if (Counter.handler) {
      clearInterval(Counter.handler);
      Counter.seconds = 0;
    }

    Counter.handler = setInterval(() => {
      Counter.seconds++;

      Counter.listeners
        .filter(listener => listener.type === "onTick")
        .forEach(listener => listener.callback(Counter.seconds));

      if (
        Counter.seconds === Counter.end &&
        Counter.listeners.length !== 0
      ) {
        Counter.listeners
          .filter(listener => listener.type === "onComplete")
          .forEach(listener => listener.callback());
        Counter.stop();
      }
    }, 1000);
  },
  stop: () => {
    if (Counter.handler) {
      clearInterval(Counter.handler);
    }
  },
  listeners: []
};

// Example
Counter.listeners.push({
  type: "onComplete",
  callback: () => console.log("Done!")
});
Counter.listeners.push({
  type: "onTick",
  callback: seconds => console.log(seconds)
});