why giveing extra element when use for...in javascript

Question

I am totally confused about for...in loop in javascript. I iterate an nodeList. but it giving me some extra element why

In HTML

<div class="mm">
    klsjdfljsdf
</div>

<div class="mm">
    klsjdfljsdf
</div>

In JS

var mm = document.querySelectorAll(".mm");
for(var i in mm){
    alert(i);
}

see this link : https://plnkr.co/edit/AtxPb4IoTpdnSYDk6jc1?p=preview