Objects in Javascript

Objects, in JavaScript, are the most important data type and form the building blocks for modern JavaScript. These objects are quite different from JavaScript’s primitive data types (Number, String, Boolean, null, undefined, and symbol) in the sense that these primitive data types all store a single value each (depending on their types).

Syntax:

new Object(value)
Object(value)
let object_name = {
    key_name : value,
    ...
}

Note:- Object()  can be called with or without new. Both create a new object.

Example: Below is an example of Objects in JavaScript.

Javascript

const o = new Object();
o.foo = 42;

console.log(o);
// { foo: 42 }

Output

{ foo: 42 }

Example: In this example “name”, “location”, and “established” are all “keys” and “Vivekananda School”, “Delhi” and 1971 are values of these keys respectively. Each of these keys is referred to as properties of the object. An object in JavaScript may also have a function as a member, in which case it will be known as a method of that object. Here  “displayinfo” is a method of the school object that is being used to work with the object’s data, stored in its properties.

javascript

// JavaScript code demonstrating a simple object
let school = {
    name: 'Vivekananda School',
    location: 'Delhi',
    established: '1971',
    displayInfo: function () {
        console.log(`${school.name} was established
              in ${school.established} at ${school.location}`);
    }
}
school.displayInfo();

Output

Vivekananda School was established 
              in 1971 at Delhi

• Objects are more complex and each object may contain any combination of these primitive data-types as well as reference data-types.
• An object is a reference data type. Variables that are assigned a reference value are given a reference or a pointer to that value. That reference or pointer points to the location in memory where the object is stored. The variables don’t actually store the value.
• Loosely speaking, objects in JavaScript may be defined as an unordered collection of related data, of primitive or reference types, in the form of “key: value” pairs. These keys can be variables or functions and are called properties and methods, respectively, in the context of an object.

An object can be created with figure brackets {…} with an optional list of properties. A property is a “key: value” pair, where a key is a string (also called a “property name”), and the value can be anything.

JavaScript Object Properties

The property names can be strings or numbers. In case the property names are numbers, they must be accessed using the “bracket notation” like this.

Example: Below is the example of object properties.

javascript

let school = {
    name: 'Vivekananda School',
    location: 'Delhi',
    established: '1971',
    20: 1000,
    displayInfo: function () {
        console.log(`The value of the key 20 is ${school['20']}`);
    }
}
school.displayInfo();

Output

The value of the key 20 is 1000

But more on the bracket notation later. Property names can also be strings with more than one space separated words. In which case, these property names must be enclosed in quotes :

let school = {
    "school name" : "Vivekananda School",
}

Example: In this example, we are accessing ‘Vivekananda’ from ‘Vivekananda School’.

javascript

// Bracket notation
let school = {
    name: 'Vivekananda School',
    displayInfo: function () {
        console.log(`${school.name.split(' ')[0]}`);
    }
}
school.displayInfo(); // Vivekananda

Output

Vivekananda

In the above code, we made use of bracket notation and also split method provided by JavaScript which you will learn about in the strings article.

Inherited Properties

Inherited properties of an object are those properties that have been inherited from the object’s prototype, as opposed to being defined for the object itself, which is known as the object’s Own property. To verify if a property is an object’s Own property, we can use the hasOwnProperty method. Property Attributes Data properties in JavaScript have four attributes.

• value: The property’s value.
• writable: When true, the property’s value can be changed
• enumerable: When true, the property can be iterated over by “for-in” enumeration. Otherwise, the property is said to be non-enumerable.
• configurable: If false, attempts to delete the property, change the property to be an access-or property, or change its attributes (other than [[Value]], or changing [[Writable]] to false) will fail.

Example: Below is the example of inherited properties.

javascript

// hasOwnProperty code in js
const object1 = new Object();
object1.property1 = 42;

console.log(object1.hasOwnProperty('property1')); // true

Output

true

Creating Objects: For creating objects refer to the following article. Ref:- https://www.geeksforgeeks.org/?p=190694

Accessing Object Members

Object members(properties or methods) can be accessed using the dot notation

(objectName.memberName);

Example: Below is the example of accessing object members.

javascript

let school = {
    name : "Vivekanada",
    location : "Delhi",
    established : 1971,
    20 : 1000,
    displayinfo : function() {
        console.log(`${school.name} was established
        in ${school.established} at ${school.location}`);
    }

}
console.log(school.name);

console.log(school.established);

Output

Vivekanada
1971

Bracket Notation

Syntax:

 objectName["memberName"]

Example: Below is the example of Bracket Notation.

javascript

let school = {
    name: "Vivekanada School",
    location: "Delhi",
    established: 1995,
    20: 1000,
    displayinfo: function () {
        document.write(`${school.name} was established
        in ${school.established} at ${school.location}`);
    }
}

// Output : Vivekanada School
console.log(school['name']);

// Output: 1000
console.log(school['20']);

Output

Vivekanada School
1000

Unlike dot notation, the bracket keyword works with any string combination, including, but not limited to multi-word strings.

somePerson.first name // invalid
somePerson["first name"] // valid

Unlike dot notation, bracket notation can also contain names that are the results of any expressions variables whose values are computed at run-time.

let key = "first name" somePerson[key] = "Name Surname"

Similar operations are not possible while using the dot notation.

Iterating over all keys of an object

To iterate over all existing enumerable keys of an object, we may use the for…in construct. It is worth noting that this allows us to access only those properties of an object which are enumerable (Recall that enumerable is one of the four attributes of data properties). For instance, properties inherited from the Object.prototype are not enumerable. But, enumerable properties inherited from somewhere can also be accessed using the for…in construct

Example: Below is the example.

javascript

let person = {
    gender: "male"
}

let person1 = Object.create(person);
person1.name = "Adam";
person1.age = 45;
person1.nationality = "Australian";

for (let key in person1) {
    // Output : name, age, nationality
    // and gender
    console.log(key);
}

Output

name
age
nationality
gender

Deleting Properties

To Delete a property of an object we can make use of the delete operator. An example of its usage has been listed below.

Example: Below is the example of deleting properties.

javascript

let obj1 = {
    propfirst : "Name"
}

// Output : Name
console.log(obj1.propfirst);
delete obj1.propfirst

// Output : undefined
console.log(obj1.propfirst);

Output

Name
undefined

Example: In this example, we can not delete inherited properties or non-configurable properties

javascript

let obj1 = {
    propfirst : "Name"
}
// Output : Name
console.log(obj1.propfirst)
let obj2 = Object.create(obj1);

// Output : Name
console.log(obj2.propfirst);

// Output : true.
console.log(delete obj2.propfirst);

    // Surprisingly Note that this will return true
    // regardless of whether the deletion was successful

    // Output : Name
    console.log(obj2.propfirst);

Output

Name
Name
true
Name