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You Don't Know JS Yet: Objects & Classes - 2nd Edition

Chapter 2: How Objects Work

Work in progress

Objects are not just containers for multiple values, though clearly that's the context for most interactions with objects.

To fully understand the object mechanism in JS, and get the most out of using objects in our programs, we need to look more closely at a number of characteristics of objects (and their properties) which can affect their behavior when interacting with them.

These characteristics that define the underlying behavior of objects are collectively referred to in formal terms as the "metaobject protocol" (MOP)1. The MOP is useful not only for understanding how objects will behave, but also for overriding the default behaviors of objects to bend the language to fit our program's needs more fully.

Property Descriptors

Each property on an object is internally described by what's known as a "property descriptor". This is, itself, an object (aka, "metaobject") with several properties (aka "attributes") on it, dictating how the target property behaves.

We can retrieve a property descriptor for any existing property using Object.getOwnPropertyDescriptor(..) (ES5):

myObj = {
    favoriteNumber: 42,
    isDeveloper: true,
    firstName: "Kyle"

// {
//     value: 42,
//     enumerable: true,
//     writable: true,
//     configurable: true
// }

We can even use such a descriptor to define a new property on an object, using Object.defineProperty(..) (ES5):

anotherObj = {};

    value: 42,
    enumerable: true,     // default if omitted
    writable: true,       // default if omitted
    configurable: true    // default if omitted

anotherObj.fave;          // 42

If an existing property has not already been marked as non-configurable (with configurable: false in its descriptor), it can always be re-defined/overwritten using Object.defineProperty(..).

A number of earlier sections in this chapter refer to "copying" or "duplicating" properties. One might assume such copying/duplication would be at the property descriptor level. However, none of those operations actually work that way; they all do simple = style access and assignment, which has the effect of ignoring any nuances in how the underlying descriptor for a property is defined.

Though it seems far less common out in the wild, we can even define multiple properties at once, each with their own descriptor:

anotherObj = {};

    "fave": {
        // a property descriptor
    "superFave": {
        // another property descriptor

It's not very common to see this usage, because it's rarer that you need to specifically control the definition of multiple properties. But it may be useful in some cases.

Accessor Properties

A property descriptor usually defines a value property, as shown above. However, a special kind of property, known as an "accessor property" (aka, a getter/setter), can be defined. For these a property like this, its descriptor does not define a fixed value property, but would instead look something like this:

    get() { .. },    // function to invoke when retrieving the value
    set(v) { .. },   // function to invoke when assigning the value
    // .. enumerable, etc

A getter looks like a property access (obj.prop), but under the covers it invokes the get() method as defined; it's sort of like if you had called obj.prop(). A setter looks like a property assignment (obj.prop = value), but it invokes the set(..) method as defined; it's sort of like if you had called obj.prop(value).

Let's illustrate a getter/setter accessor property:

anotherObj = {};

    get() { console.log("Getting 'fave' value!"); return 123; },
    set(v) { console.log(`Ignoring ${v} assignment.`); }

// Getting 'fave' value!
// 123

anotherObj.fave = 42;
// Ignoring 42 assignment.

// Getting 'fave' value!
// 123

Enumerable, Writable, Configurable

Besides value or get() / set(..), the other 3 attributes of a property descriptor are (as shown above):

  • enumerable
  • writable
  • configurable

The enumerable attribute controls whether the property will appear in various enumerations of object properties, such as Object.keys(..), Object.entries(..), loops, and the copying that occurs with the ... object spread and Object.assign(..). Most properties should be left enumerable, but you can mark certain special properties on an object as non-enumerable if they shouldn't be iterated/copied.

The writable attribute controls whether a value assignment (via =) is allowed. To make a property "read only", define it with writable: false. However, as long as the property is still configurable, Object.defineProperty(..) can still change the value by setting value differently.

The configurable attribute controls whether a property's descriptor can be re-defined/overwritten. A property that's configurable: false is locked to its definition, and any further attempts to change it with Object.defineProperty(..) will fail. A non-configurable property can still be assigned new values (via =), as long as writable: true is still set on the property's descriptor.

Object Sub-Types

There are a variety of specialized sub-types of objects in JS. But by far, the two most common ones you'll interact with are arrays and functions.

By "sub-type", we mean the notion of a derived type that has inherited the behaviors from a parent type but then specialized or extended those behaviors. In other words, values of these sub-types are fully objects, but are also more than just objects.


Arrays are objects that are specifically intended to be numerically indexed, rather than using string named property locations. They are still objects, so a named property like favoriteNumber is legal. But it's greatly frowned upon to mix named properties into numerically indexed arrays.

Arrays are preferably defined with literal syntax (similar to objects), but with the [ .. ] square brackets rather than { .. } curly brackets:

myList = [ 23, 42, 109 ];

JS allows any mixture of value types in arrays, including objects, other arrays, functions, etc. As you're likely already aware, arrays are "zero-indexed", meaning the first element in the array is at the index 0, not 1:

myList = [ 23, 42, 109 ];

myList[0];      // 23
myList[1];      // 42

Recall that any string property name on an object that "looks like" an integer -- is able to be validly coerced to a numeric integer -- will actually be treated like an integer property (aka, integer index). The same goes for arrays. You should always use 42 as an integer index (aka, property name), but if you use the string "42", JS will assume you meant that as an integer and do that for you.

// "2" works as an integer index here, but it's not advised
myList["2"];    // 109

One exception to the "no named properties on arrays" rule is that all arrays automatically expose a length property, which is automatically kept updated with the "length" of the array.

myList = [ 23, 42, 109 ];

myList.length;   // 3

// "push" another value onto the end of the list

myList.length;   // 4
Many JS developers incorrectly believe that array length is basically a getter (see "Accessor Properties" earlier in this chapter), but it's not. The offshoot is that these developers feel like it's "expensive" to access this property -- as if JS has to on-the-fly recompute the length -- and will thus do things like capture/store the length of an array before doing a non-mutating loop over it. This used to be "best practice" from a performance perspective. But for at least 10 years now, that's actually been an anti-pattern, because the JS engine is more efficient at managing the length property than our JS code is at trying to "outsmart" the engine to avoid invoking something we think is a getter. It's more efficient to let the JS engine do its job, and just access the property whenever and however often it's needed.

Empty Slots

JS arrays also have a really unfortunate "flaw" in their design, referred to as "empty slots". If you assign an index of an array more than one position beyond the current end of the array, JS will leave the in between slots "empty" rather than auto-assigning them to undefined as you might expect:

myList = [ 23, 42, 109 ];
myList.length;              // 3

myList[14] = "Hello";
myList.length;              // 15

myList;                     // [ 23, 42, 109, empty x 11, "Hello" ]

// looks like a real slot with a
// real `undefined` value in it,
// but beware, it's a trick!
myList[9];                  // undefined

You might wonder why empty slots are so bad? One reason: there are APIs in JS, like array's map(..), where empty slots are suprisingly skipped over! Never, ever intentionally create empty slots in your arrays. This in undebateably one of JS's "bad parts".


I don't have much specifically to say about functions here, other than to point out that they are also sub-object-types. This means that in addition to being executable, they can also have named properties added to or accessed from them.

Functions have two pre-defined properties you may find yourself interacting with, specifially for meta-programming purposes:

function help(opt1,opt2,...remainingOpts) {
    // ..
};          // "help"
help.length;        // 2

The length of a function is the count of its explicitly defined parameters, up to but not including a parameter that either has a default value defined (e.g., param = 42) or a "rest parameter" (e.g., ...remainingOpts).

Avoid Setting Function-Object Properties

You should avoid assigning properties on function objects. If you're looking to store extra information associated with a function, use a separate Map(..) (or WeakMap(..)) with the function object as the key, and the extra information as the value.

extraInfo = new Map();

extraInfo.set(help,"this is some important information");

// later:
extraInfo.get(help);   // "this is some important information"

Object Characteristics

In addition to defining behaviors for specific properties, certain behaviors are configurable across the whole object:

  • extensible
  • sealed
  • frozen


Extensibility refers to whether an object can have new properties defined/added to it. By default, all objects are extensible, but you can change shut off extensibility for an object:

myObj = {
    favoriteNumber: 42

myObj.firstName = "Kyle";                  // works fine


myObj.nicknames = [ "getify", "ydkjs" ];   // fails
myObj.favoriteNumber = 123;                // works fine

In non-strict-mode, an assignment that creates a new property will silently fail, whereas in strict mode an exception will be thrown.





Extending The MOP

As mentioned at the start of this chapter, objects in JS behave according to a set of rules referred to as the Metaobject Protocol (MOP)1. Now that we understand more fully how objects work by default, we want to turn our attention to how we can hook into some of these default behaviors and override/customize them.


[[Prototype]] Chain

One of the most important, but least obvious, characteristics of an object (part of the MOP) is referred to as its "prototype chain"; the official JS specification notation is [[Prototype]]. Make sure not to confuse this [[Prototype]] with a public property named prototype. Despite the naming, these are distinct concepts.

The [[Prototype]] is an internal linkage that an object gets by default when its created, pointing to another object. This linkage is a hidden, often subtle characteristic of an object, but it has profound impacts on how interactions with the object will play out. It's referred to as a "chain" because one object links to another, which in turn links to another, ... and so on. There is an end or top to this chain, where the linkage stops and there's no further to go. More on that shortly.

We already saw several implications of [[Prototype]] linkage in Chapter 1. For example, by default, all objects are [[Prototype]]-linked to the built-in object named Object.prototype.

That Object.prototype name itself can be confusing, since it uses a property called prototype. How are [[Prototype]] and prototype related!? Put such questions/confusion on pause for a bit, as we'll come back an explain the differences between [[Prototype]] and prototype later in this chapter. For the moment, just assume the presence of this important but weirdly named built-in object, Object.prototype.

Let's consider some code:

myObj = {
    favoriteNumber: 42

That should look familiar from Chapter 1. But what you don't see in this code is that the object there was automatically linked (via its internal [[Prototype]]) to that automatically built-in, but weirdly named, Object.prototype object.

When we do things like:

myObj.toString();                             // "[object Object]"

myObj.hasOwnProperty("favoriteNumber");   // true

We're taking advantage of this internal [[Prototype]] linkage, without really realizing it. Since myObj does not have toString or hasOwnProperty properties defined on it, those property accesses actually end up DELEGATING the access to continue its lookup along the [[Prototype]] chain.

Since myObj is [[Prototype]]-linked to the object named Object.prototype, the lookup for toString and hasOwnProperty properties continues on that object; and indeed, these methods are found there!

The ability for myObj.toString to access the toString property even though it doesn't actually have it, is commonly referred to as "inheritance", or more specifically, "prototypal inheritance". The toString and hasOwnProperty properties, along with many others, are said to be "inherited properties" on myObj.

I have a lot of frustrations with the usage of the word "inheritance" here -- it should be called "delegation"! -- but that's what most people refer to it as, so we'll begrudgingly comply and use that same terminology for now (albeit under protest, with " quotes). I'll save my objections for an appendix of this book.

Object.prototype has several built-in properties and methods, all of which are "inherited" by any object that is [[Prototype]]-linked, either directly or indirectly through another object's linkage, to Object.prototype.

Some common "inherited" properties from Object.prototype include:

  • constructor
  • __proto__
  • toString()
  • valueOf()
  • hasOwnProperty(..)
  • isPrototypeOf(..)

Recall hasOwnProperty(..), which we saw earlier gives us a boolean check for whether a certain property (by string name) is owned by an object:

myObj = {
    favoriteNumber: 42

myObj.hasOwnProperty("favoriteNumber");   // true

It's always been considered somewhat unfortunate (semantic organization, naming conflicts, etc) that such an important utility as hasOwnProperty(..) was included on the Object [[Prototype]] chain as an instance method, instead of being defined as a static utility.

As of ES2022, JS has finally added the static version of this utility: Object.hasOwn(..).

myObj = {
    favoriteNumber: 42

Object.hasOwn(myObj,"favoriteNumber");   // true

This form is now considered the more preferable and robust option, and the instance method (hasOwnProperty(..)) form should now generally be avoided.

Somewhat unfortunately and inconsisently, there's not (yet, as of time of writing) corresponding static utilities, like Object.isPrototype(..) (instead of the instance method isPrototypeOf(..)). But at least Object.hasOwn(..) exists, so that's progress.

Creating An Object With A Different [[Prototype]]

By default, any object you create in your programs will be [[Prototype]]-linked to that Object.prototype object. However, you can create an object with a different linkage like this:

myObj = Object.create(differentObj);

The Object.create(..) method takes its first argument as the value to set for the newly created object's [[Prototype]].

One downside to this approach is that you aren't using the { .. } literal syntax, so you don't initially define any contents for myObj. You typically then have to define properties one-by-one, using =.

The second, optional argument to Object.create(..) is -- like the second argument to Object.defineProperties(..) as discussed earlier -- an object with properties that hold descriptors to initially define the new object with. In practice out in the wild, this form is rarely used, likely because it's more awkward to specify full descriptors instead of just name/value pairs. But it may come in handy in some limited cases.

Alternately, but less preferably, you can use the { .. } literal syntax along with a special (and strange looking!) property:

myObj = {
    __proto__: differentObj,

    // .. the rest of the object definition
The strange looking __proto__ property has been in some JS engines for more than 20 years, but was only standardized in JS as of ES6 (in 2015). Even still, it was added in Appendix B of the specification2, which lists features that TC39 begrudgingly includes because they exist popularly in various browser-based JS engines and therefore are a de-facto reality even if they didn't originate with TC39. This feature is thus "guaranteed" by the spec to exist in all conforming browser-based JS engines, but is not necessarily guaranteed to work in other independent JS engines. Node.js uses the JS engine (v8) from the Chrome browser, so Node.js gets __proto__ by default/accident. Be careful when using __proto__ to be aware of all the JS engine environments your code will run in.

Whether you use Object.create(..) or __proto__, the created object in question will usually be [[Prototype]]-linked to a different object than the default Object.prototype.

Empty [[Prototype]] Linkage

We mentioned above that the [[Prototype]] chain has to stop somewhere, so as to have lookups not continue forever. Object.prototype is typically the top/end of every [[Prototype]] chain, as its own [[Prototype]] is null, and therefore there's nowhere else to continue looking.

However, you can also define objects with their own null value for [[Prototype]], such as:

emptyObj = Object.create(null);
// or: emptyObj = { __proto__: null }

emptyObj.toString;   // undefined

It can be quite useful to create an object with no [[Prototype]] linkage to Object.prototype. For example, as mentioned in Chapter 1, the in and constructs will consult the [[Prototype]] chain for inherited properties. But this may be undesirable, as you may not want something like "toString" in myObj to resolve successfully.

Moreover, an object with an empty [[Prototype]] is safe from any accidental "inheritance" collision between its own property names and the ones it "inherits" from elsewhere. These types of (useful!) objects are sometimes referred to in popular parlance as "dictionary objects".

[[Prototype]] vs prototype

Notice that public property name prototype in the name/location of this special object, Object.prototype? What's that all about?

Object is the Object(..) function; by default, all functions (which are themselves objects!) have such a prototype property on them, pointing at an object.

Any here's where the name conflict between [[Prototype]] and prototype really bites us. The prototype property on a function doesn't define any linkage that the function itself experiences. Indeed, functions (as objects) have their own internal [[Prototype]] linkage somewhere else -- more on that in a second.

Rather, the prototype property on a function refers to an object that should be linked TO by any other object that is created when calling that function with the new keyword:

myObj = {};

// is basically the same as:
myObj = new Object();

Since the { .. } object literal syntax is essentially the same as a new Object() call, the built-in object named/located at Object.prototype is used as the internal [[Prototype]] value for the new object we create and name myObj.

Phew! Talk about a topic made significantly more confusing just because of the name overlap between [[Prototype]] and prototype!

But where do functions themselves (as objects!) link to, [[Prototype]] wise? They link to Function.prototype, yet another built-in object, located at the prototype property on the Function(..) function.

In other words, you can think of functions themselves as having been "created" by a new Function(..) call, and then [[Prototype]]-linked to the Function.prototype object. This object contains properties/methods all functions "inherit" by default, such as toString() (to string serialize the source code of a function) and call(..) / apply(..) / bind(..) (we'll explain these later in this book).

Objects Behavior

Properties on objects are internally defined and controlled by a "descriptor" metaobject, which includes attributes such as value (the property's present value) and enumerable (a boolean controlling whether the property is included in enumerable-only listings of properties/property names).

The way object and their properties work in JS is referred to as the "metaobject protocol" (MOP)1. We can control the precise behavior of properties via Object.defineProperty(..), as well as object-wide behaviors with Object.freeze(..). But even more powerfully, we can hook into and override certain default behaviors on objects using special pre-defined Symbols.

Prototypes are internal linkages between objects that allow property or method access against one object -- if the property/method requested is absent -- to be handled by "delegating" that access lookup to another object. When the delegation involves a method, the context for the method to run in is shared from the initial object to the target object via the this keyword.


  1. "Metaobject", Wikipedia; ; Accessed July 2022. 2 3

  2. "Appendix B: Additional ECMAScript Features for Web Browsers", ECMAScript 2022 Language Specification; ; Accessed July 2022