JavaScript: The Definitive Guide, 6th Edition

Enumerating Properties

Instead of testing for the existence of individual properties, we sometimes want to iterate through or obtain a list of all the properties of an object. This is usually done with the for/in loop, although ECMAScript 5 provides two handy alternatives.

The for/in loop was covered in for/in. It runs the body of the loop once for each enumerable property (own or inherited) of the specified object, assigning the name of the property to the loop variable. Built-in methods that objects inherit are not enumerable, but the properties that your code adds to objects are enumerable (unless you use one of the functions described later to make them nonenumerable). For example:

var o = {x:1, y:2, z:3};             // Three enumerable own properties
o.propertyIsEnumerable("toString")   // => false: not enumerable
for(var p in o)                      // Loop through the properties
    console.log(p);                  // Prints x, y, and z, but not toString

Some utility libraries add new methods (or other properties) to Object.prototype so that they are inherited by, and available to, all objects. Prior to ECMAScript 5, however, there is no way to make these added methods nonenumerable, so they are enumerated by for/in loops. To guard against this, you might want to filter the properties returned by for/in. Here are two ways you might do so:

for(p in o) {
    if (!o.hasOwnProperty(p)) continue;       // Skip inherited properties
}

for(var p in o) {
    if (typeof o[p] === "function") continue; // Skip methods
}

Example 6-2 defines utility functions ...