button onclick function

document.getElementById("myBtn").addEventListener("click", function() {
  alert("Hello World!");
});

<button onclick="myFunction()">Click me</button>

<p id="demo"></p>

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

var button = document.querySelector('button');
button.onclick = function() {
  //do stuff
}

var myElem = document.getElementByID('ElemID');
myElem.onclick = function() {
	//do stuff
}

<button onclick="myFunction()">Click me</button>
 

<button onClick="script">