index.html

    <div class="container">
        <div class="wrapper">
            <h1 class="header" id="header">This is Header</h1>
            <a href="#" id="anchor" class="anchor-link">CLICK ME</a>
            <p>lorempixel lorempixel lorempixel lorempixel lorempixel lorempixel lorempixel lorempixel lorempixel lorempixel lorempixel lorempixel lorempixel lorempixel lorempixel
                lorempixel lorempixel lorempixel lorempixel lorempixel
                lorempixel lorempixel lorempixel lorempixel lorempixel
            </p>
            <a href="#" id="anchor" class="anchor-link">CLICK ME</a>
            <h1 class="header" id="header">This is Header</h1>
            <a href="#" id="anchor" class="anchor-link">CLICK ME</a>
        </div>
    </div>

script.js

var headerId = document.getElementById('header');
console.log('headerId => ', headerId) // HTML Collection
// If header element appears more than one, headerClass will be of length `1` only. 

var headerClass = document.getElementsByClassName('header');
console.log('headerClass => ', headerClass); // Node List
// If header element appears more than one, headerClass will be of length `2` 

// Access desired header element
var headerClassFirst = document.getElementsByClassName('header')[0];
console.log('headerClassFirst => ', headerClassFirst); // HTML Collection

var headerQuerySelector = document.querySelector('.header');
console.log('headerQuerySelector => ', headerQuerySelector); // HTML Collection

var headerQuerySelectorAll = document.querySelectorAll('.header');
console.log('headerQuerySelectorAll => ', headerQuerySelectorAll); // Node List
// If header element appears more than one, headerQuerySelectorAll will be of length `2`

var anchorTagName = document.getElementsByTagName('a');
console.log('anchorTagName => ', anchorTagName); // Node List
// If anchor element appears more than one, anchorTagName will be of length `2` or `more`

// References: https://plainjs.com/javascript/selecting/