javascript-for-in.js

var prop, x;
var animal = {
  type: 'Missing No.',
  say: function (phrase) {
    return this.type + ' says hello.';
  }
};

var dog = Object.create(animal);
dog.type = 'Dog';
dog.bark = function () {
  return 'WOOF! ' + this.say() + ' WOOF!';
}

var poodle = Object.create(dog);
poodle.bark = function () {
  return 'bark. ' + this.say() + ' bark.';
}

// log the properties in animal first
for (prop in animal) {
  console.log(prop);    // > say, type
}

// now log the properties that for...in sees in poodle
for (prop in poodle) {
  console.log(prop);    // > bark, say, type
  // but we only defined bark directly on poodle
}

// if we only want the properties directly on poodle, we can use Object.hasOwnProperty
for (prop in poodle) {
  if (poodle.hasOwnProperty(prop)) {
    console.log(prop);  // > bark
  }
}


// We can use for...in on arrays as well, but it probably doesn't do what you'd expect (also, it doesn't guarantee order)
var someArray = ['a', 'b', 'c'];
for (x in someArray) {
  console.log(x);             // > 0, 1, 2
}
// you could use this (order is not guaranteed)
for (x in someArray) {
  console.log(someArray[x]);  // 'a', 'b', 'c'
}
// but.... you really should be using this
someArray.forEach(function (y) {
  console.log(y);             // 'a', 'b', 'c'
});