What does enumerable mean?

I was directed to MDN's for..in page when it said, "for..in Iterates over the enumerable properties of an object."

Then I went to the Enumerability and ownership of properties page where it said "Enumerable properties are those which can be iterated by a for..in loop."

The dictionary defines enumerable as countable, but I can't really visualize what that means. Could i get an example of something being enumerable?

 

Well, whether a property is considered enumerable or not is based on its own [[Enumerable]] attribute. You can view this as part of the property's descriptor:

var descriptor = Object.getOwnPropertyDescriptor({ bar: 1 }, 'bar');

console.log(descriptor.enumerable); // true
console.log(descriptor.value);      // 1

console.log(descriptor);
// { value: 1, writable: true, enumerable: true, configurable: true }

> var descriptor = Object.getOwnPropertyDescriptor({ bar: 1 }, 'bar');
undefined
>
undefined
> console.log(descriptor.enumerable); // true
true
undefined
> console.log(descriptor.value);      // 1
1
undefined
>
undefined
> console.log(descriptor);
{ value: 1, writable: true, enumerable: true, configurable: true }
undefined
> // { value: 1, writable: true, enumerable: true, configurable: true }
undefined
>

A for..in loop then iterates through the object's property names.

var foo = { bar: 1, baz: 2};

for (var prop in foo)
    console.log(prop); // outputs 'bar' and 'baz'

> var foo = { bar: 1, baz: 2};
undefined
>
undefined
> for (var prop in foo)
...     console.log(prop); // outputs 'bar' and 'baz'
bar
baz
undefined

But, it only evaluates its statement -- console.log(prop); in this case -- for those properties whose [[Enumerable]] attribute is true.

This condition is in place because objects actually have many more properties, especially those from inheritance:

> console.log(Object.getOwnPropertyNames(Object.prototype));
[ 'constructor',
  'toString',
  'toLocaleString',
  'valueOf',
  'hasOwnProperty',
  'isPrototypeOf',
  'propertyIsEnumerable',
  '__defineGetter__',
  '__lookupGetter__',
  '__defineSetter__',
  '__lookupSetter__' ]
undefined
> // ["constructor", "toString", "toLocaleString", "valueOf", "hasOwnProperty",
"isPrototypeOf", "propertyIsEnumerable", /* etc. */]

console.log(Object.getOwnPropertyNames(Object.prototype));
// ["constructor", "toString", "toLocaleString", "valueOf", "hasOwnProperty", "isPrototypeOf", "propertyIsEnumerable", /* etc. */]

Each of these properties still exists on the object:

console.log('constructor' in foo); // true
console.log('toString' in foo);    // true
// etc.

But, they're skipped (or "not counted") by the for..in loop because they're non-enumerable.

var descriptor = Object.getOwnPropertyDescriptor(Object.prototype, 'constructor');

console.log(descriptor.enumerable); // false