Why is instance variable undefined?
<p>I dont understand one thing: </p>

<pre><code> var comein = document.getElementById("comein");

 var enter = new Expand({ elem : comein });

function Expand (options) {
    this._elem = options.elem;
    console.log(this._elem); // i have a div element    
}

Expand.prototype = {        
    check : function () {

        var comInClassName = this._elem.className; // i have an error: this._elem is undefined

        if (comInClassName == "open"){
          this.close();
        }
        if (comInClassName == "close"){
          this.open();
        }
    }
}
log_in.addEventListener("click",  enter.check, false);
</code></pre>

<p>Why I have an error in prototype method if in Expand i have a normal element? Thanks</p>
