how to get javaScript event source element?

Your html should be like this:

<button onclick="doSomething" id="id_button">action</button>

And renaming your input-paramter to event like this

function doSomething(event){
    var source = event.target || event.srcElement;
    console.log(source);
}

would solve your problem.

As a side note, I’d suggest taking a look at jQuery and unobtrusive javascript