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).
While these primitive data types can each hold only a single value, objects are like containers that can hold multiple values at once. These values are stored as properties of the object, each with its own key. This allows us to group related data and functions together, making our code more organised and easier to understand.
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.
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 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 or a symbol (also called a “property name”), and the value can be anything including numbers, strings, booleans, functions, arrays, or even other objects. .
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.
let school = {
name: 'Vivekananda School',
location: 'Delhi',
established: '1971',
20: 1000,
displayInfo: function () {
console.log(`The value of the key 20 is ${school['20']}`);
}
}
school.displayInfo();
Output
The value of the key 20 is 1000
But more on the bracket notation later. Property names can also be strings with more than one space separated words. In which case, these property names must be enclosed in quotes :
let school = {
"school name" : "Vivekananda School",
}
Example: In this example, we are accessing ‘Vivekananda’ from ‘Vivekananda School’.
// Bracket notation
let school = {
name: 'Vivekananda School',
displayInfo: function () {
console.log(`${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.
// 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:- Creating objects in JavaScript
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.
let school = {
name : "Vivekanada",
location : "Delhi",
established : 1971,
20 : 1000,
displayinfo : function() {
console.log(`${school.name} was established
in ${school.established} at ${school.location}`);
}
}
console.log(school.name);
console.log(school.established);
Output
Vivekanada 1971
Bracket Notation
Syntax:
objectName["memberName"]
Example: Below is the example of Bracket Notation.
let school = {
name: "Vivekanada School",
location: "Delhi",
established: 1995,
20: 1000,
displayinfo: function () {
document.write(`${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.
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.
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
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