Enumerable Properties in JavaScript

Before digging in, let's learn how to check if a property is enumerable. To do this we can use Object.prototype.propertyIsEnumerable().

Hang with me here, but JavaScript objects have a property called propertyIsEnumerable, which, as you might guess, checks to see if a property is enumerable. For fun, let's check if the propertyIsEnumerable property is an enumerable property :)

let obj = {}
obj.propertyIsEnumerable('propertyIsEnumerable')  // => false

Nope!

The above example also demonstrates the usage. Let's see a few more examples:

let obj = {
    firstname: 'Harry',
    lastname: 'Potter'
}

obj.propertyIsEnumerable('constructor')  // => false
obj.propertyIsEnumerable('__proto__')  // => false
obj.propertyIsEnumerable('firstname')  // => true
obj.propertyIsEnumerable('lastname')  // => true

Now that we know how to check if a property is enumerable, let's learn how to define enumerable properties. To do this we're going to use Object.defineProperties and Object.defineProperty.

Object.defineProperty

First, let's take an existing, empty object.

let obj = {};

Typically, we could define properties like this:

obj.someProp = 'someValue';

But if we wanted to specify internal flags (like enumerable) for this property, we can do so using defineProperty.

Object.defineProperty(obj, 'someProp', {
    value: 'someValue',
    enumerable: true
});

Now, the above example is unnecessary. It accomplished the same thing as normal property assignment because the default value for enumerable is true anyways. But I wanted to show how the enumerable property would be set, or unset.

Let's create a property that is not enumerable.

Object.defineProperty(obj, 'nonEnumerableProp', {
    value: 'someValue',
    enumerable: false
});

Now, let's prove our work:

obj.propertyIsEnumerable('someProp')  // => true
obj.propertyIsEnumerable('nonEnumerableProp')  // => false

Object.defineProperties

We can also define more than one property at a time using defineProperties. Notice it takes a different set of arguments.

Object.defineProperties(obj, {
    'someProp': {
        value: 'someValue',
        enumerable: true
    },
    'nonEnumerableProp': {
        value: 'someValue',
        enumerable: false
    }
});

One of the most important reasons to understand property enumerability is to understand how to access properties during iteration. Since JavaScript objects are complicated and have many different kinds of properties, we don't always want to access every property when looping over properties.

We've written an in-depth guide on iterating over JavaScript object properties and values, but I will provide an overview here as well.

A note on inherited properties

Another important characteristic of a property is whether or not it is an own property, meaning it belongs to the object directly, or whether it is inherited, meaning it exists on the objects prototype chain.

Take a look at the following table, to see how to access properties with various characteristics.