HTML button onclick

<button onclick="myFunction()">Click me</button>
 <!DOCTYPE html>
    <title>Title of the document</title>
    <p>There is a hidden message for you. Click to see it.</p>
    <button onclick="myFunction()">Click me!</button>
    <p id="demo"></p>
      function myFunction() {
        document.getElementById("demo").innerHTML = "Hello Dear Visitor!</br> We are happy that you've chosen our website to learn programming languages. We're sure you'll become one of the best programmers in your country. Good luck to you!";
</html>var button = document.querySelector('button');
button.onclick = function() {
  //do stuff
}<button onclick="Function()">Text</button><script>

<input type="button" value="Capacity Chart" id="MyButton" >var myElem = document.getElementByID('ElemID');
myElem.onclick = function() {
	//do stuff