Diving Deeper in JavaScripts Objects

Diving Deeper in JavaScripts Objects

JavaScript objects pack more things than their terse and concise syntax would naturally exhibit. Creating and using objects in JavaScript is so easy, effortless, and so flexible that a lot of developers never realize that there is more to it.

JavaScript objects pack more things than their terse and concise syntax would naturally exhibit. Creating and using objects in JavaScript is so easy, effortless, and so flexible that a lot of developers never realize that there is more to it.

We are going to uncover some of the hidden layers and understand the intricacies of JavaScript objects. After reading this article, you should be able to answer the following questions —

  • How to make a property undeletable?
  • What are accessor properties and what are their features?
  • How to make a property immutable or hidden?
  • Why some properties do *not appear in* for-in loops or Object.keys and some do?
  • How to “protect” objects from modification?
  • How to make sense of code such as
obj.id = 5;
console.log(obj.id)
// => '101' ( 5 in binary )

Types of Properties

Data Properties

You must have made countless objects like these

const obj = {
  name: 'Arfat',
  id: 5
}

obj.name 
// => 'Arfat'

In object obj, name and id properties are called Data Properties. These are the normal kind of properties that constitute most of the JavaScript code. What is, then, the other type of property?

Accessor Properties

They can also be understood as getters and setters of other languages such as C# or Python. An accessor property is a combination of *two functions: the* get and the set function.

Instead of using the traditional key: valuesyntax, we use the following syntax —

const accessorObj = {
  get name() {
    return 'Arfat';
  }
};

accessorObj.name;
// => 'Arfat'

const dataObj = {
  name: 'Arfat',
};

dataObj.name;
// => 'Arfat'

Look at accessorObj and compare it with dataObj. Right now, they exhibit the same behavior. We use the get keyword and follow it with a function declaration. Reading accessor properties do not need to use parentheses to invoke the function. That is, accessorObj.name(); is wrong.

When we access(read) accessorObj.name, the name function is executed and its return value becomes the final value of the name key.

The get function is called a getter since it is involved in getting the value of a key. If you update accessorObj.name = 'New Person’;, the update won’t happen. This is because we don’t have a corresponding setter function for the name key. Setter functions help in setting values of getter properties.

const accessorObj = {
  _name: 'Arfat',
  get name() {
    return this._name;
  },
  set name(value) {
    this._name = value;
  }
};

The *setter function receives the assigned value as a parameter.* Now, you can store the value in property or a global variable. In this case, we make a conventional “private” property _name and store the name value in it.

In the getter function, we can modify or override the property before returning its value. The following example should demonstrate this. It also answers one of the above questions.

const obj = {
  get name() {
    return this._name.toUpperCase();
  },
  set name(value) {
    this._name = value;
  },
  get id() {
    return this._id.toString(2); // Returns binary of a number
  },
  set id(value) {
    this._id = value;
  }
}

obj.name = 'Arfat';
obj.name;
// => 'ARFAT'

obj.id = 5;
obj.id;
// => '101

Why would anybody use accessor properties if normal data properties exist? There are often cases where you need to log the property access or maintain a history of all the values that the property has had. Accessor properties give us the full power of a function with the ease of use of object properties. You can read more about accessor usage here.

So how does JavaScript know which is an accessor property and which is a data property? Let’s find out.

Object Property Descriptors

At first glance, it might look like there is a one-to-one mapping between keys and values of an object. However, that’s not entirely true.

Property Attributes

Every key of an object contains a set of property attributes that define the characteristics of the value associated with the key. They can also be thought of as meta-data describing the key-value pair. In short, attributes are used to define and explain the state of object properties.

The set of property attributes is called the property descriptor.

In total, there are six property attributes. They are —

  • How to make a property undeletable?
  • What are accessor properties and what are their features?
  • How to make a property immutable or hidden?
  • Why some properties do *not appear in* for-in loops or Object.keys and some do?
  • How to “protect” objects from modification?
  • How to make sense of code such as

Why have we wrapped the attribute names in [[]]? Double square brackets mark internal properties used by the ECMA specifications. These are properties that JS programmer cannot touch directly from within the code. To manipulate internal properties, we’d need to use methods provided to us by the language.

Let’s see an example —

In the above image, the object has 2 keys: x and y. You can see the corresponding list of attributes associated with each property.

How can you get the same information in JavaScript? We can use the function Object.getOwnPropertyDescriptor to get that information. It takes an object and a property name and returns an object containing the required attributes. Here’s a code sample —

const object = {
  x: 5,
  y: 6
};

Object.getOwnPropertyDescriptor(object, 'x');

/* 
{ 
  value: 5, 
  writable: true, 
  enumerable: true, 
  configurable: true 
}
*/

Let’s see the attributes in more detail to know how they are helpful and then we’ll revisit the image.

[[Value]]

It stores the value retrieved by a get access of the property. Which means that when we do object.x in the above example, we actually retrieve its [[Value]] attribute. Any dot-access or square-bracket access of a Data property will work in this way.

[[Get]]

It stores the reference to the function that we declare while making a *getter property.* It is called with an empty arguments list to retrieve the property value each time a get access of the property is performed.

[[Set]]

It stores the reference to the function that we declare while making a setter property. It is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed.

const obj = {
  set x(val) {
    console.log(val) 
    // => 23
  }
}

obj.x = 23;

In the above example, the right-hand-side of the assignment is passed as the val argument to a setter function. You can see this code for a demonstration.

[[Writable]]

This is a boolean value. It tells whether we can overwrite the value or not. If false, attempts to change the property’s value will not succeed.

[[Enumerable]]

This is also a boolean value. This attribute dictates whether the property is going to appear in for-in loops or not. If true, the for-in loop will be able to iterate on this property.

[[Configurable]]

This is a boolean too.

When it is false —

  • How to make a property undeletable?
  • What are accessor properties and what are their features?
  • How to make a property immutable or hidden?
  • Why some properties do *not appear in* for-in loops or Object.keys and some do?
  • How to “protect” objects from modification?
  • How to make sense of code such as

The effect of this property is also dependent on the property type. Apart from the above effects, it also does the following.

  • How to make a property undeletable?
  • What are accessor properties and what are their features?
  • How to make a property immutable or hidden?
  • Why some properties do *not appear in* for-in loops or Object.keys and some do?
  • How to “protect” objects from modification?
  • How to make sense of code such as

All six properties do not exist for each property type.

  • How to make a property undeletable?
  • What are accessor properties and what are their features?
  • How to make a property immutable or hidden?
  • Why some properties do *not appear in* for-in loops or Object.keys and some do?
  • How to “protect” objects from modification?
  • How to make sense of code such as

    Working with Descriptors

Having learned about these descriptors, how can we set or update them on our own objects? There are a couple of functions in JavaScript that can be used to work with these descriptors. Let’s see them —

Object.getOwnPropertyDescriptor

We have seen this function above. It takes an existing object, and a property name. It returns either undefined or an object containing the descriptors.

Object.defineProperty

It’s a static method on Object that can define or modify a new property on a given object. It takes three arguments — the object, the property name, and descriptors. It returns the modified object. Let’s see an example —

const obj = {};

Object.defineProperty(obj, 'id', {
  value: 42
});

console.log(obj);
// => { }

console.log(obj.id);
// => 42

Object.defineProperty(obj, 'name', {
  value: 'Arfat',
  writable: false,
  enumerable: true,
  configurable: true
});

console.log(obj.name);
// => 'Arfat'

obj.name = 'Arfat Salman'

console.log(obj.name);
// => 'Arfat' 
// (instead of 'Arfat Salman')

Object.defineProperty(obj, 'lastName', {
  value: 'Salman',
  enumerable: false,
});

console.log(Object.keys(obj));
// => [ 'name' ]

delete obj.id;

console.log(obj.id);
// => 42

Object.defineProperties(obj, {
  property1: {
    value: 42,
    writable: true
  },
  property2: {}
});

console.log(obj.property1)
// => 42

It seems like a long example but it’s simple. Let’s go step by step —

  • How to make a property undeletable?
  • What are accessor properties and what are their features?
  • How to make a property immutable or hidden?
  • Why some properties do *not appear in* for-in loops or Object.keys and some do?
  • How to “protect” objects from modification?
  • How to make sense of code such as

Object.defineProperty sets one property at a time. There is a different variant of this function that can set multiple properties with their descriptors at the same time. It is aptly named Object.defineProperties](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperties) "https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperties)")[.](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperties ".")

Protecting Objects

There are often cases where we don’t want anyone to tamper with our objects. Given the flexibility of JavaScript, it’s really easy to mistakenly re-assign properties of objects that we are not meant to touch. There are three major ways of protecting objects in JavaScript. Let’s look at them —

Object.preventExtensions

The Object.preventExtensions method prevents new properties from ever being added to an object (i.e. prevents future extensions to the object). It takes an object and makes it non-extensible.

Note that the properties *can be deleted though.*

const obj = {
  id: 42
};

Object.preventExtensions(obj);

obj.name = 'Arfat';

console.log(obj);
// => { id: 42 } 

You can check whether an object is non-extensible or not by using Object.isExtensible. If it returns true, you can add more properties to the object.

Object.seal

The seal method seals an object. It means —

  • How to make a property undeletable?
  • What are accessor properties and what are their features?
  • How to make a property immutable or hidden?
  • Why some properties do *not appear in* for-in loops or Object.keys and some do?
  • How to “protect” objects from modification?
  • How to make sense of code such as
const obj = {
  id: 42
};

Object.seal(obj);

delete obj.id 
// (does not work)

obj.name = 'Arfat';
// (does not work)

console.log(obj);
// => { id: 42 }

Object.isExtensible(obj);
// => false

Object.isSealed(obj);
//=> true

You can use Object.isSealed to test whether an object has been sealed or not.

Object.freeze

freeze is the maximum protection any object can have in JavaScript. It —

  • How to make a property undeletable?
  • What are accessor properties and what are their features?
  • How to make a property immutable or hidden?
  • Why some properties do *not appear in* for-in loops or Object.keys and some do?
  • How to “protect” objects from modification?
  • How to make sense of code such as
const obj = {
  id: 42
};

Object.freeze(obj);

delete obj.id 
// (does not work)

obj.name = 'Arfat';
// (does not work)

console.log(obj);
// // => { id: 42 }

Object.isExtensible(obj);
// // => false

Object.isSealed(obj);
// //=> true

Object.isFrozen(obj);
// => true

You can check whether an object is frozen or not using the Object.isFrozen function.

An important point to note is that these methods deal only with the direct properties of the objects. They do not modify nested objects.

Here’s a table summarizing the same.

The operations are in terms of properties. [CREATE] a new property, [READ] an existing property, [UPDATE] an existing property and [DELETE] an existing property.

Conclusion

Given how pervasive objects are on JavaScripts, it is important to understand the true power of objects. I hope the article was able to convey that effectively to you. I also hope that now you are able to answer the questions listed at the beginning of the article. Thanks for reading.

Learn More

The Complete JavaScript Course 2018: Build Real Projects!

Become a JavaScript developer - Learn (React, Node,Angular)

JavaScript: Understanding the Weird Parts

Vue JS 2 - The Complete Guide (incl. Vue Router & Vuex)

The Full JavaScript & ES6 Tutorial - (including ES7 & React)

JavaScript - Step By Step Guide For Beginners

The Web Developer Bootcamp

MERN Stack Front To Back: Full Stack React, Redux & Node.js

Angular 9 Tutorial: Learn to Build a CRUD Angular App Quickly

What's new in Bootstrap 5 and when Bootstrap 5 release date?

Brave, Chrome, Firefox, Opera or Edge: Which is Better and Faster?

How to Build Progressive Web Apps (PWA) using Angular 9

What is new features in Javascript ES2020 ECMAScript 2020

JavaScript Tutorial: if-else Statement in JavaScript

This JavaScript tutorial is a step by step guide on JavaScript If Else Statements. Learn how to use If Else in javascript and also JavaScript If Else Statements. if-else Statement in JavaScript. JavaScript's conditional statements: if; if-else; nested-if; if-else-if. These statements allow you to control the flow of your program's execution based upon conditions known only during run time.

How to Retrieve full Profile of LinkedIn User using Javascript

I am trying to retrieve the full profile (especially job history and educational qualifications) of a linkedin user via the Javascript (Fetch LinkedIn Data Using JavaScript)

Java vs. JavaScript: Know The Difference

Java vs. JavaScript: Know the Difference, Java vs. JavaScript: What's the Difference? Java vs. JavaScript: Major Similarities and Differences. pros and cons of JavaScript and Java.