Array.prototype.slice()

arraySlice.js

//Array.prototype.slice()
var nodeList = decument.getElementsByTagname('span');
alert(nodeList instanceof NodeList); // => true
alert(nodeList instanceof Array); // => false
var array = Array.prototype.slice.call(nodeList);

alert(array instanceof NodeList); // => false
alert(array instanceof Array);  // => true

FYI: 日本語解説
https://developer.mozilla.org/ja/docs/JavaScript/Reference/Global_Objects/Array/slice