I am converting JavaScript object to array but in result I am getting one `null`

here my code

```
<div id="a"></div>

Object.prototype.toArray = function(){
    var arr = [];
    for (var i in this) {
        arr.push(this[i]);        
    }
    return arr;
} 

var providers = {
            facebooklike: "Facebook Like",
            facebookrecommend : "Facebook Recommend",
            facebooksend : "Facebook Send",
            twittertweet : "Twitter Tweet",
            linkedinshare : "LinkedIn Share",
            linkedinrecommend : "LinkedIn Recommend",
            googleplusplusone : "Google+ +1",
            googleplusshare : "Google+ Share"
        };

var a = document.getElementById("a");
a.innerHTML = JSON.stringify(providers.toArray());

```

My result is 

> 
> ["Facebook Like","Facebook Recommend","Facebook Send","Twitter
>  Tweet","LinkedIn Share","LinkedIn Recommend","Google+ +1","Google+
>  Share",null]
> 
> 
> 

Here is fiddle [example](http://jsfiddle.net/govindmalviya/Fy26h/1/)