JavaScript 中的对象是什么？

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

  user.isadmin = true // adding
  delete user.age // removing

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

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

  function makeuser(name, age) {
    return { name, age } // same as name: name, age: age
  }

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

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

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

let userprofile = {
  firstname: 'jane',
  lastname: 'smith',
  email: 'jane.smith@example.com',
  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('jane.doe@example.com') // output: email updated to jane.doe@example.com

// accessing nested properties
console.log(userprofile.address.city) // output: metropolis

// iterating over interests
console.log('user interests:')
userprofile.interests.foreach((interest) => console.log(interest))

// 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

let key = 'favoritecolor'
let userpreferences = {
  [key]: 'blue',
  [key + 'secondary']: 'green',
}

console.log(userpreferences.favoritecolor) // output: blue
console.log(userpreferences.favoritecolorsecondary) // output: green

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

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()

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