What are Objects in JavaScript?

• Definition: Objects store keyed collections of data and more complex entities.
• Creation:
  • Object Constructor Syntax: let user = new Object();
  • Object Literal Syntax: let user = {}; (preferred and widely used).

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Literals and Properties

• Objects are collections of properties. A property is a key: value pair.

  let user = {
    name: 'John',
    age: 30,
  }

• Accessing Properties:
  • Dot Notation: user.name returns "John".
  • Square Bracket Notation: user["name"] also returns "John".
• Adding/Removing Properties:

  user.isAdmin = true // Adding
  delete user.age // Removing

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Special Cases with Keys

• Multiword Keys: Use quotes and square brackets.

  user['likes birds'] = true
  alert(user['likes birds']) // true

• Dynamic Keys (Computed Properties):
  • You can use variables or expressions as keys.

  let fruit = 'apple'
  let bag = { [fruit]: 5 } // Equivalent to { "apple": 5 }

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Shorthand Properties

• When a variable name matches the property name:

  function makeUser(name, age) {
    return { name, age } // Same as name: name, age: age
  }

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Property Name Rules

• Object properties can use reserved words or special characters.
• Non-string keys (e.g., numbers) are converted to strings:

  let obj = { 0: 'test' }
  alert(obj[0]) // "test"

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Testing and Iterating Properties

1. Property Existence:
   • Use "key" in obj to check if a key exists.

   let user = { age: undefined }
   alert('age' in user) // true

1. Iterating with for..in:

   let user = { name: 'John', age: 30 }
   for (let key in user) {
     alert(key) // Outputs: "name", "age"
     alert(user[key]) // Outputs: "John", 30
   }

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Property Order

• Integer keys: Sorted in ascending order.
• Non-integer keys: Retain their creation order.

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Realistic Code Sample: A User Profile

let userProfile = {
  firstName: 'Jane',
  lastName: 'Smith',
  email: '[e-mail]',
  isVerified: true,
  address: {
    street: '123 Elm Street',
    city: 'Metropolis',
    postalCode: '12345',
  },
  interests: ['reading', 'hiking', 'coding'],

  // Method inside an object
  getFullName() {
    return `${this.firstName} ${this.lastName}`
  },

  // Dynamically updating properties
  updateEmail(newEmail) {
    this.email = newEmail
    console.log(`Email updated to ${this.email}`)
  },
}

// Accessing properties
console.log(userProfile.getFullName()) // Output: Jane Smith

// Updating email using the method
userProfile.updateEmail('[e-mail]') // Output: Email updated to [e-mail]

// Accessing nested properties
console.log(userProfile.address.city) // Output: Metropolis

// Iterating over interests
console.log('User Interests:')
userProfile.interests.forEach((interest) => console.log(interest))

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Adding and Deleting Properties

Properties can be added or removed dynamically after an object is created.

// Adding a new property
userProfile.phoneNumber = '555-1234'
console.log(userProfile.phoneNumber) // Output: 555-1234

// Deleting a property
delete userProfile.isVerified
console.log(userProfile.isVerified) // Output: undefined

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Computed Properties

When creating objects, the property name can be dynamically computed using square brackets.

let key = 'favoriteColor'
let userPreferences = {
  [key]: 'blue',
  [key + 'Secondary']: 'green',
}

console.log(userPreferences.favoriteColor) // Output: blue
console.log(userPreferences.favoriteColorSecondary) // Output: green

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Iterating Over Object Properties

Using for...in, you can loop through all keys in an object.

for (let key in userProfile) {
  console.log(`${key}: ${userProfile[key]}`)
}

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Realistic Example: Product Inventory

Here’s how objects can be used in a practical scenario, such as managing a product inventory:

let inventory = {
  products: [
    {
      id: 1,
      name: 'Laptop',
      price: 1200,
      stock: 25,
    },
    {
      id: 2,
      name: 'Smartphone',
      price: 800,
      stock: 50,
    },
    {
      id: 3,
      name: 'Tablet',
      price: 400,
      stock: 30,
    },
  ],

  // Method to display all products
  listProducts() {
    console.log('Product Inventory:')
    this.products.forEach((product) => {
      console.log(
        `${product.name} - $${product.price} (Stock: ${product.stock})`
      )
    })
  },

  // Method to update stock
  updateStock(productId, quantity) {
    let product = this.products.find((p) => p.id === productId)
    if (product) {
      product.stock += quantity
      console.log(`${product.name} stock updated to ${product.stock}`)
    } else {
      console.log('Product not found!')
    }
  },
}

// List products
inventory.listProducts()

// Update stock
inventory.updateStock(2, -5) // Decrease stock for Smartphone
inventory.listProducts()

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Using in Operator

The in operator checks if a property exists in an object. It’s particularly useful when verifying optional or dynamically added properties.

if ('phoneNumber' in userProfile) {
  console.log('Phone number exists:', userProfile.phoneNumber)
} else {
  console.log('Phone number not available.')
}

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Summary

Objects are central to JavaScript, offering flexibility and functionality:

• Keys can be strings or symbols.
• Access properties using dot or bracket notation.
• Use for..in to iterate over keys.
• Understand integer vs. non-integer property ordering.