where to add javascript file in html code example

Example 1: how to open html file with javascript

// In my case, I used a button to activate this function:
function myFunction() {
    window.location.href = 'index.html';
}

Example 2: how to link javascript to html

<script src = "index.js"></script>

Example 3: how to add script in html head

<script src="script.js" defer></script>

<!--just add defer attribute so script will execute at last-->

Example 4: how to include external javascript in html

<script src="script.js">