querySelectorAll()の戻り値はArrayじゃないよ ref: http://qiita.com/naosim_/items/f489e200f4e648be2f53

file0.js

var divList = document.querySelectorAll('div');
var a = divList[0];// 1つ目の要素
var b = divList[1];// 2つ目の要素

file1.js

console.log(divList instanceof Array);// false
divList.forEach(function(v, i) { console.log(i); });// エラー

file2.js

console.log(toString.call(divList));

file3.js

function mixinArray(target) {
  var arrayMethods = Object.getOwnPropertyNames(Array.prototype);
  arrayMethods.forEach(attachArrayMethodsToNodeList);
  function attachArrayMethodsToNodeList(methodName) {
    if(methodName !== "length") {
      target[methodName] = Array.prototype[methodName];
    }
  };
}

mixinArray(NodeList.prototype);

file4.js

mixinArray(divList);