🌐 User Defined Objects in JavaScript

(Creating Objects, Instances, Objects within Objects)

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🧠 1. Introduction to User Defined Objects

In JavaScript, a User Defined Object is an object created by the programmer to store and manage related data and behavior.

👉 Objects consist of:

• Properties (data)
• Methods (functions)

✅ Example:

let student = {
  name: "Rahul",
  age: 22,
  greet: function() {
    console.log("Hello " + this.name);
  }
};

student.greet();

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🧩 2. Creating a User Defined Object

There are multiple ways to create objects:

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(1) Object Literal (Most Common)

let person = {
  name: "Aman",
  age: 25,
  city: "Delhi"
};

✔ Advantages:

• Simple and quick
• Easy to read

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(2) Using new Object()

let person = new Object();
person.name = "Aman";
person.age = 25;

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(3) Using Constructor Function

function Person(name, age) {
  this.name = name;
  this.age = age;
}

let p1 = new Person("Rahul", 22);
let p2 = new Person("Aman", 25);

👉 This is used to create multiple similar objects.

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(4) Using ES6 Class

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }
}

let p1 = new Person("Rahul", 22);

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🧬 3. Instances of Objects

An instance is a specific object created from a constructor or class.

🔹 Example:

function Car(model) {
  this.model = model;
}

let c1 = new Car("BMW");   // Instance 1
let c2 = new Car("Audi");  // Instance 2

📌 Key Points:

• Each instance has its own data
• Instances share structure (same constructor)

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🔄 4. Adding Properties and Methods Dynamically

Objects in JavaScript are dynamic.

let user = {};

user.name = "John";
user.age = 30;

user.greet = function() {
  console.log("Hi " + this.name);
};

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🧩 5. Objects within Objects (Nested Objects)

JavaScript allows objects to contain other objects.

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🔹 Example:

let student = {
  name: "Rahul",
  address: {
    city: "Delhi",
    pincode: 110001
  }
};

console.log(student.address.city);

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🔹 Complex Example:

let company = {
  name: "TechCorp",
  employee: {
    name: "Aman",
    skills: {
      primary: "JavaScript",
      secondary: "Python"
    }
  }
};

console.log(company.employee.skills.primary);

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🔍 6. Accessing Object Properties

(a) Dot Notation

console.log(student.name);

(b) Bracket Notation

console.log(student["name"]);

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🔄 7. Iterating Through Object

for (let key in student) {
  console.log(key + ": " + student[key]);
}

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🔄 8. Complete Example

function Student(name, age) {
  this.name = name;
  this.age = age;
  this.display = function() {
    console.log(this.name + " - " + this.age);
  };
}

let s1 = new Student("Rahul", 22);
let s2 = new Student("Aman", 25);

s1.display();
s2.display();

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🔄 Summary Table

Concept	Description
User Defined Object	Custom object created by user
Object Literal	Simple object creation
Constructor	Create multiple objects
Instance	Object created from constructor
Nested Object	Object inside object

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⚠️ Important Points

• Objects are dynamic and flexible
• Use constructor or class for multiple objects
• Nested objects help represent real-world structures

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🎯 Conclusion

User Defined Objects are essential for:

• Structuring data
• Creating reusable components
• Building real-world applications

👉 They are a core concept in object-oriented programming in JavaScript and very important for MCA exams.

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