<button id="btnSubmit" value="Submit">Submit</button>
External script file
var btn = document.querySelector('#btnSubmit');
btn.addListener('click', function(){
alert('Loading...');
}, false);
I get
null is not an object evaluating btn.addEventListener
It is reading before the button is rendered.
It however works fine in jsfiddle. http://jsfiddle.net/7enwttgq/