An object in JavaScript is a collection of key-value pairs. Each key-value pair is called as a property. A property can be a function, an array, an object itself or any primitive data type i.e. integer, string, etc. Functions in object are called as methods.

Consider a simple object:

var human = {
	firstName: "Virat",
	lastName: "Kohli",
	age: 30,
	fullName: function(){
		return this.firstName + " " + this.lastName		
	}
}

Here firstName, lastName, and fullName are properties of the same object i.e. human. Every human will have these properties but their values may be different i.e. firstName, lastName may have different value for different human.

console.log(human);

1. Dot notation
2. Square bracket notation

human.firstName; //Output: Virat

human.fullName(); //Output: Virat Kohli

New properties can be added using the dot notation as shown below:

human.age = 27
human.getAge = function(){
	return this.age;
}

Now console human and we will find new properties age and getAge() added to the human object as shown below:

Console.log(human);

human["firstName"]; //Output: Virat

human["fullName"](); //Output: Virat Kohli

New properties can be added using the Square bracket notation as shown below:

human["weight"] = 65
human.getWeight = function(){
	return this.weight;
}

Properties can also be accessed using a variable having value equal to the property name as shown below:

var firstNameProperty = "firstName";

console.log(human[firstNameProperty]) // Output: Virat

Note: Above method of using variable to access property names cannot be used to access properties of the object using dot notation

Console.log(human.firstNameProperty) //Output: undefined

Note: If we try to access the methods without using the (), output will be method definition as shown below:

console.log(human.fullName); //Output: function definition

An object property name can be any valid JavaScript string, or anything that can be converted to a string, including the empty string. However, any property name that is not a valid Javascript identifier (for example, a property name that has a space or a hyphen, or that starts with a number) can only be accessed and added to the object property using the square bracket notation

human["date of birth"] = "Nov 28";
human[12] = 12;
human.12 = 12; //gives error
console.log(human);

console.log(human.12); //Gives error
console.log(human[12]); //Output: 12

human object created above is an example of creating an object using object literal

var human = {
	firstName: "Virat",
	lastName: "Kohli",
	age: 30,
	fullName: function(){
		return this.firstName + " " + this.lastName		
	}
}

var human = new Object()
console.log(human);// Creates an empty object

We can added as many properties as we want usign either the dot notation or the square bracket notation

human.firstName = "Virat";
human.lastName = "Kohli";
human.age = 30;
human.fullName = function(){
	return this.firstName + " " + this.lastName;
}
console.log(human)

Method 1 and 2 do exactly the same thing. There is no need to use new Object(). For simplicity, readability and execution speed, use the first one that is object literal.

Objects can be created using the constructor function using the following two steps:

1. Define the object type by writing the constructor function. By convention, name of the constructor function starts with a capital letter
2. Create an instance of the object with new

To define an object type, create a function for the object type that specifies its name, properties, and methods. Lets create constructor function for the human type object

function Human(firstName, lastName){
	this.firstName = firstName,
	this.lastName = lastName,
	this.fullName = function(){
		return this.firstName + " " + this.lastName;
	}
}

Now you can create as many objects as you want using this constructor function:

var viratKohli = new Human("Virat", "Kohli");
console.log(viratKohli);

var sachinTendulkar = new Human("Sachin", "Tendulkar");
console.log(sachinTendulkar);

To delete a property from an object we can use the ‘delete’ operator. You cannot delete properties that were inherited, nor can you delete properties with their attributes set to configurable.

‘delete’ operator returns true if the delete was successful. It also return true if the property to delete was non-existent or the property could not be deleted

delete human.firstName; // return true
console.log(human);

Let's see what happens if we try to call fullName method which uses both the firstName and lastName property of human object

console.log(human.fullName());// undefined Kohli

Output is undefined because we were trying to access firstName property of human object which does not exists