Sometime, use can rewrite the toString , valueOf method to make those function more useful:

For exmaple, we can make valueOf() function to calcualte the sum, and then use toString method to display the information of the object we create.

var Tornado = function(category, affectedAreas, windGust){    this.category = category;    this.affectedAreas = affectedAreas;    this.windGust = windGust;};var cities = [["Kansas City", 46310],["Topeka", 127939],["Lenexa", 49398]];var twister = new Tornado("F5", cities, 220);cities.push(["Olathe", 130045]);twister.toString();Tornado.prototype.toString = function(){    var list = "";    for(var i = 0; i< this.affectedAreas.length; i++){        if(i < this.affectedAreas.length-1){            list = list + this.affectedAreas[i][0] + ", ";        }else{            list = list + "and "+ this.affectedAreas[i][0];        }    }    return "This tornado has been classified as an " + this.category+    ", with wind gusts up to "+ this.windGust+ "mph. Affected areas are:"+    list+", potentially affecting a population of "+ this.valueOf() + ".";};Tornado.prototype.valueOf = function(){    var sum = 0;    for(var i = 0; i < this.affectedAreas.length; i++){        sum += this.affectedAreas[i][1];    }    return sum;}Object.prototype.findOwnProperty = function(propName){    var currentObject = this;    while(currentObject !== null){        if(currentObject.hasOwnProperty(propName)){            return currentObject;        }else{            currentObject = currentObject.__proto__;        }    }    return "No property found!";}twister.findOwnProperty("valueOf");