Why does he give arguments to the function at the end?

At about 2 mins in the video, you can see that the "attachListItemButtons" function takes one parameter named "li", which represents the element that the new buttons are added to.

As you can see attachListItemsButtons accept an argument li , which will be the current li item where the buttons are added to. Now whenever you will call the attachListItemsButtons , you have to provide it an argument , so he does. lis[i] represents a single li element on which button is added. Hope it helps.

But why???

I get that he gives the "li" argument, but I don't get the reason...

I simplified the example to test if you had to give the li (as in list element) for the argument, but it works with anything there.

Is it kinda like the event object?

      <ul>
        <li>one</li>
        <li>two</li>
        <li>three</li>
        <li>four</li>
      </ul>

const parent = document.getElementsByTagName('ul')[0];
const child = parent.children;



const newButtons = whatever => {

  let button = document.createElement('button');

      button.className = 'button';
      button.textContent = 'Button';

  whatever.appendChild(button);

}

for (let i = 0; i < child.length; i += 1) {
  newButtons(child[i]);
}

You can check it in https://codepen.io/danvao/pen/ERPxOq