The .getElementsByTagName() method returns a NodeList object, or an array of elements inside an element (or the tag name that you have provided to the method).
For example, the below script will return a NodeList object, which will have all <li> element on a web page.
<script> document.getElementsByTagName('li'); </script>
Try it
Since, I am using the method getElementsByTagName() with the document, it will return all the <li> elements (visible or invisible) on a web page. You can iterate the array (the NodeList object) and get the details of each element.
To get all the lis inside an ul element, you have to provide the parent <ul> element to the method. For example,
<ul id='nav_1'> <li>t1</li> <li>t2</li> <li>t3</li> </ul> <script> let lis = document.getElementById('nav_1').getElementsByTagName('li');