how to use button in javascript code example

Example 1: javascript onclick

document.getElementById("Save").onclick = function ()
    {
     alert("hello");
     //validation code to see State field is mandatory.  
    }

Example 2: javascript onclick

// The element id (TheElementID) and var name (myElement) can be named anything.
var myElement = document.getElementByID('TheElementID');
myElement.onclick = function() {
	// Carry out a function here...
}