Java Script in TinyMCE

I have a simple slider function in JavaScript, and it works on online compliers, but it seems like none of my JS code will work in Tiny.
Is there something I am missing, or is it even possible to use JS with Tiny?
Thank you
Fareed Khamitov

It is possible to use javascript. Please include more details about what you are doing and the problem you are encountering.

<h2>JavaScript in Body</h2>

<p id="demo">A Paragraph.</p>

<button type="button" onclick="myFunction()">Try it</button>

<script>
function myFunction() {
document.getElementById("demo").innerHTML = "Paragraph changed.";
}
</script>

This is the tester function I got from W3schools to test. Should change "A paragaraph" to "Paragraph Changed" after a click on the button. It does work on online html complier and the W3 complier, but when I use it in tiny the button is not clickable. Same problem was with the slider, the arrows were non-responsive.

Thank you.

Please provide the URL so that we can inspect the code.

http://fakes.greatmockup.com/tester/