Cannot read properties of undefined trigger when trying to click upon window load


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
  const now =;
  const then = now + seconds * 1000;
function startTimer() {
  const seconds = parseInt(this.dataset.time);
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')

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.

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