JavaScript Array对象扩展indexOf()方法

背景：JavaScript中Array对象的标准方法中，没有indexOf()方法，可通过下面的代码扩展。

if (!Array.prototype.indexOf) { 
Array.prototype.indexOf = function(elt) { 
var len = this.length >>> 0; 
var from = Number(arguments[1]) || 0; 
from = (from < 0) ? Math.ceil(from) : Math.floor(from); 
if (from < 0) from += len; 
for (; from < len; from++) { 
if (from in this && this[from] === elt) return from; 
} 
return - 1; 
}; 
}