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

> var descriptor = Object.getOwnPropertyDescriptor({ bar: 1 }, ‘bar‘);undefined>undefined> console.log(descriptor.enumerable); // truetrueundefined> console.log(descriptor.value);      // 11undefined>undefined> console.log(descriptor);{ value: 1, writable: true, enumerable: true, configurable: true }undefined> // { value: 1, writable: true, enumerable: true, configurable: true }undefined>

Afor..inLoop 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‘barbazundefined

But, it only evaluates itsStatement--console.log(prop);In this case -- for those properties whose[[Enumerable]]Attribute istrue.

This condition is in place because objects actually have has 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); // trueconsole.log(‘toString‘ in foo);    // true// etc.

But, they're skipped (or"Not counted") Byfor..inLoop because they're non-enumerable.

var descriptor = Object.getOwnPropertyDescriptor(Object.prototype, ‘constructor‘);console.log(descriptor.enumerable); // false

 

What does enumerable mean?