addEventListener with inexplicable undefined method error

I couldn't find an answer to th开发者_运维百科is problem I've been having.

function UploadBar() {
    this.reader = new FileReader();
    this.reader.addEventListener(
        "onloadstart"
        , function(evt) {alert("Hello World");}
        , false
    );
}

when I try to run this code it gives me an undefined_method_error in the javascript debugger in chrome. Could anyone be so kind as to tell me what is wrong here?

---

reader is not an element, so don't use .addEventListener Instead do the following.

function UploadBar() {
    this.reader = new FileReader();
    this.reader.onloadstart = function(e) { alert('Hello World') };  
}