JavaScript Array對象擴展indexOf()方法

背景：JavaScript中Array對象的標準方法中，沒有indexOf()方法，可通過下面的代碼擴展。

複製代碼代碼如下:

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;

};

}