I would like to trigger a timer to run upon my page loading for the first time.

This is what I have tried:

let countdown;
...
const buttons = document.querySelectorAll("[data-time]");
...
function timer(seconds) {
  // clear any existing timers
  clearInterval(countdown);
  const now = Date.now();
  const then = now + seconds * 1000;
  displayTimeLeft(seconds);
  displayEndTime(then);
...
function startTimer() {
  const seconds = parseInt(this.dataset.time);
  timer(seconds);
}
...
window.addEventListener('load', buttons[0].addEventListener('click', startTimer).trigger('click'))

I get the error:

Uncaught TypeError: Cannot read properties of undefined (reading 'trigger')

>Solution :

The easiest way would be doing like so :

window.addEventListener('load', startTimer)

I am not sure why you want to trigger the click event on the button, but if you have to, you should do something like that :

window.addEventListener('load', () => {
   buttons[0].addEventListener('click', startTimer)
   const event = new Event('click')
   buttons[0].dispatchEvent(event)
})

The error you have is due to the fact that addEventListener doesn’t return anything having a method called trigger (it doesn’t return anything at all).

But again, it should be enough to pass startTimer as a callback to the load event listener, no need to nest an additional event listener and trigger the click event on it.