How to add a onclick event to an element using javascript

Try this:
document.getElementsByClassName always returns array of elements.

 var element= document.getElementsByClassName('classname');
 for(var i=0;i<element.length;i++){
      element[i].addEventListener("click", function(){alert('clicked')}, false);   
 }

You have to pass reference of a function instead of adding inline alert.

var element= document.getElementsByClassName('classname');
function doSomething() {
  alert('clicked')
}

// add event listener to element 
element.addEventListener("click", doSomething, false);

getElementsByClassName returns an HTMLCollection, so even though you have only one element with that classname in DOM, you have to retrieve it with index 0:

var element = document.getElementsByClassName('classname')[0];
element.addEventListener("click", function(e) {
    alert('something');
}, false);

Alternatively, since you only have one element with the classname, you can safely use querySelector, which will return the first match element.

var element = document.querySelector('.classname');
                                      ^
element.addEventListener("click", function(e) {
    alert('something');
}, false);

Please note the dot in above code. querySelector accepts a CSS selector string as a parameter.

You can use .attr() jquery method for this:

$('.classname').attr("onClick", "javascript:alert('clicked'); return false;"); 

Try this

JavaScript

var element= document.getElementsByClassName('classname');
element[0].onclick = function() { alert('Hello'); };//-- here i used "[0]" to refer first matched element 

Try with javascript