Refaktoryzacja Kodu

var a0 = document.getElementById('a0');
var a1 = document.getElementById(a1');
var a2 = document.getElementById('a2');
....
....
....
....itd

a0.addEventListener("click", function() { dupaFun(0); });
a1.addEventListener("click", function() { dupaFun(1); });
a2.addEventListener("click", function() { dupaFun(2); });
...
...
...
...itd

for (i=0; i<=iloscDup; i++)
{
	var a+i = document.getElementById('a'+i);
	a+i.addEventListener("click", function() { dupaFun(i); });
}

<button data-type="a" data-value="1">Click me!</button>
<button data-type="a" data-value="2">Click me!</button>
<button data-type="a" data-value="3">Click me!</button>

const buttons = document.querySelectorAll('[data-type=a]')

for (let i = 0; i < buttons.length; i++) {
  buttons[i].addEventListener('click', (event) => {
    console.log(event.target.dataset.value)
  })
}

[...document.querySelectorAll('[data-type=a]')]
  .forEach(button => button.addEventListener('click', ({ target }) => {
    console.log(target.dataset.value)
  }))

<div id="container">
  <button data-type="a" data-value="1">Click me!</button>
  <button data-type="a" data-value="2">Click me!</button>
  <button data-type="a" data-value="3">Click me!</button>
</div>

document
  .getElementById('container')
  .addEventListener('click', ({ target }) => {
    if (target.dataset.type === 'a') {
      console.log(target.dataset.value)
    }
  })

<button id="a1">Click me!</button>
<button id="a2">Click me!</button>
<button id="a3">Click me!</button>

for (let i = 1; i <= 3; i++) {
  document
    .getElementById(`a${i}`)
    .addEventListener('click', () => {
      console.log(i)
    })
}