Objects, Arrays of Objects & Objects of Arrays

3.1 What is an Object?

An object is a collection of named properties. Each property has a key (property name) and a value.

let person = {
  name: "Alice",
  age: 25,
  isStudent: true
};

3.2 Object Literal Syntax

You create an object using curly braces { }. Inside them, properties are written as key : value pairs separated by commas.

let person = {
  name: "Alice",      // line 2
  age: 25,            // line 3
  isStudent: true     // line 4
};

console.log(person.name); // line 7
console.log(person.age);  // line 8

How the example works

1. let person = { starts an object literal assigned to person.
2. name: "Alice" creates a property named name with the value "Alice".
3. age: 25 creates an age property with a numeric value.
4. isStudent: true creates an isStudent property with a Boolean value.
5. } ends the object literal.
6. person.name uses dot notation to access the name property.
7. person.age uses dot notation to access the age property.

3.3 Accessing and Modifying Properties

JavaScript provides two common ways to access object properties:

You can also add new properties or change existing properties after the object has been created.

let car = {
  brand: "Toyota",
  model: "Corolla"
};

console.log(car.brand);        // line 5
console.log(car["model"]);     // line 6

car.year = 2020;               // line 8
car["color"] = "blue";         // line 9

console.log(car.year);         // line 11
console.log(car.color);        // line 12

What happens here?

• car.brand accesses brand using dot notation.
• car["model"] accesses model using bracket notation.
• car.year = 2020 adds a new year property.
• car["color"] = "blue" adds a new color property.

3.4 Objects Containing Multiple Types

Object property values can be different JavaScript data types. The source material highlights strings, numbers, booleans, arrays, other objects, and functions (methods).

let book = {
  title: "JavaScript Guide",
  pages: 300,
  available: true
};

console.log(book.title);     // line 6
console.log(book.pages);     // line 7
console.log(book.available); // line 8

Here, title stores a string, pages stores a number, and available stores a Boolean value.

4. Arrays of Objects and Objects of Arrays

Real-world data often requires combining arrays and objects. This lets us represent collections of similar records or group related lists under named keys.

4.1 Array of Objects

An array of objects is useful when you have a list of similar entities, each with named fields.

let students = [
  { name: "Alice", age: 20 },  // line 2
  { name: "Bob", age: 22 },    // line 3
  { name: "Cara", age: 21 }     // line 4
];

console.log(students[0].name); // line 7
console.log(students[1].age);  // line 8

Understanding the access pattern

1. students is an array.
2. Each { ... } is an object representing one student.
3. students[0].name gets the first object and then reads its name.
4. students[1].age gets the second object and then reads its age.

let students = [
  { name: "Alice", age: 20 },
  { name: "Bob", age: 22 },
  { name: "Cara", age: 21 }
];

for (let i = 0; i < students.length-1; i++) {
  console.log(students[i].name + " is " + students[i].age);
}

The loop accesses each student object through students[i], then reads its name and age properties.

let students = [
  {name:'bob', branch:"CSE-AIML", fee:350000},
  {name:'ben', branch:"CSE", fee:300000},
  {name:'bint', branch:"CSE-DS", fee:300000}
];

for(let value of students){
  for(let key in value){
    console.log(key+" is "+value[key]);
  }
  console.log("----------------");
}

4.2 Object of Arrays

An object of arrays is useful when you group several related lists under named keys.

let course = {
  titles: ["Math", "Physics", "Chemistry"],   // line 2
  credits: [3, 4, 3]                          // line 3
};

console.log(course.titles[1]);   // line 6
console.log(course.credits[1]);  // line 7

How it works

• course is an object with two properties.
• titles is an array containing course names.
• credits is an array containing credit values.
• course.titles[1] accesses "Physics".
• course.credits[1] accesses 4, the credits for Physics.

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