JavaScript Patterns 2.4 For-in loop

javaScript Patterns 2.4 For-in loop

PRinciple

1. Enumeration should be used to iterate over nonarray objects.
2. It's important to use the method hasOwnProperty()when iterating over object properties to filter out properties that come down the prototype chain.

// the objectvar man = {    hands: 2,    legs: 2,    heads: 1};// somewhere else in the code// a method was added to all objectsif (typeof Object.prototype.clone = = = "undefined") {    Object.prototype.clone = function () {};}// 1. for-in loopfor (var i in man) {    if (man.hasOwnProperty(i)) { // filter        console.log(i, ":", man[i]);    }}/*    result in the console    hands : 2    legs : 2    heads : 1*/// 2. antipattern:// for-in loop without checking hasOwnProperty()for (var i in man) {    console.log(i, ":", man[i]);}/*    result in the console    hands : 2    legs : 2    heads : 1    clone: function()*/

Call method off of the Object.prototype to avoid naming collisions that man object redefined hasOwnProperty. And use a local variable to cache it.

var i,    hasOwn = Object.prototype.hasOwnProperty;    for (i in man) {        if (hasOwn.call(man, i)) { // filter            console.log(i, ":", man[i]);    }}