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HTML

              
                Try changing out igQuery to igGetElems in the loop.
<div id="posts">
	<div class="ig-post">asdasd</div>
	<div class="ig-post">asdasd</div>
	<div class="ig-post">asdasd</div>
	<div class="ig-post">asdasd</div>
	<div class="ig-post">asdasd</div>
</div>


Note: This is a very basic example, and you could move the code around in such a way that both methods will work.

Sometimes in real life though, this is not always as easy to see or do when there are many lines of code in the way. This is why it's good to know the difference between these two methods so you can debug quicker.
              
            
!

CSS

              
                body {
	padding: 20px;
	max-width: 500px;
	margin: auto;
}

#posts {
	border: 1px solid currentcolor;
	margin: 20px 0;
	padding: 20px;
}
              
            
!

JS

              
                let igGetElems = document.getElementsByClassName('ig-post'); 
let igQuery = document.querySelectorAll('.ig-post');

//append some items to the DOM
var div = document.createElement("div");
div.setAttribute("class", "ig-post");
div.innerText = 'appended item';
document.querySelector('#posts').appendChild(div);

//attach some event to the items
for(var i = 0; i < igQuery.length ; i++) {
  igQuery[i].addEventListener('click', function() {
    this.setAttribute("style", "background-color: red;");
  });
}

console.log(igGetElems,igQuery);

              
            
!
999px

Console