What are useful JavaScript methods that extends built-in objects? [closed]

Answer 1

Array.prototype.indexOf = Array.prototype.indexOf || function (item) {
    for (var i=0; i < this.length; i++) {
        if(this[i] === item) return i;
    }
    return -1;
};

Usage:

var list = ["my", "array", "contents"];
alert(list.indexOf("contents"));     // outputs 2