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30 Days Of JavaScript

_{Author:
Asabeneh Yetayeh

January, 2020}

📔 Day 8

📔 Day 8

Scope

Variable is the fundamental part in programming. We declare variable to store different data types. To declare a variable we use the key word var, let and const. A variable can declared at different scope. In this section we will see the scope, scope of variables when we use var or let. Variables scopes can be:

Window
Global
Local

Variable can be declared globally or locally or window scope. We will see both global and local scope. Anything declared without let, var or const is scoped at window level.

Let us image we have a scope.js file.

Window Scope

Without using console.log() open your browser and check, you will see the value of a and b if you write a or b on the browser. That means a and b are already available in the window.

//scope.js
a = 'JavaScript' // is a window scope this found anywhere
b = 10 // this is a window scope variable
function letsLearnScope() {
  console.log(a, b)
  if (true) {
    console.log(a, b)
  }
}
console.log(a, b) // accessible

Global scope

A globally declared variable can be accessed every where in the same file. But the term global is relative. It can be global to the file or it can be global relative to some block of codes.

//scope.js
let a = 'JavaScript' // is a global scope it will be found anywhere in this file
let b = 10 // is a global scope it will be found anywhere in this file
function letsLearnScope() {
  console.log(a, b) // JavaScript 10, accessible
  if (true) {
    let a = 'Python'
    let b = 100
    console.log(a, b) // Python 100
  }
  console.log(a, b)
}
letsLearnScope()
console.log(a, b) // JavaScript 10, accessible

Local scope

A variable declared as local can be accessed only in certain block code.

//scope.js
let a = 'JavaScript' // is a global scope it will be found anywhere in this file
let b = 10 // is a global scope it will be found anywhere in this file
function letsLearnScope() {
  console.log(a, b) // JavaScript 10, accessible
  let c = 30
  if (true) {
    // we can access from the function and outside the function but 
    // variables declared inside the if will not be accessed outside the if block
    let a = 'Python'
    let b = 20
    let d = 40
    console.log(a, b, c) // Python 20 30
  }
  // we can not access c because c's scope is only the if block
  console.log(a, b) // JavaScript 10
}
letsLearnScope()
console.log(a, b) // JavaScript 10, accessible

Now, you have an understanding of scope. A variable declared with var only scoped to function but variable declared with let or const is block scope(function block, if block, loop etc). Block in JavaScript is a code in between two curly brackets ({}).

//scope.js
function letsLearnScope() {
  var gravity = 9.81
  console.log(gravity)

}
// console.log(gravity), Uncaught ReferenceError: gravity is not defined

if (true){
  var gravity = 9.81
  console.log(gravity) // 9.81
}
console.log(gravity)  // 9.81

for(var i = 0; i < 3; i++){
  console.log(i) // 1, 2, 3
}
console.log(i)

In ES6 and above there is let and const, so you will not suffer from the sneakiness of var. When we use let our variable is block scoped and it will not infect other parts of our code.

//scope.js
function letsLearnScope() {
  // you can use let or const, but gravity is constant I prefer to use const
  const gravity = 9.81
  console.log(gravity)

}
// console.log(gravity), Uncaught ReferenceError: gravity is not defined

if (true){
  const  gravity = 9.81
  console.log(gravity) // 9.81
}
// console.log(gravity), Uncaught ReferenceError: gravity is not defined

for(let i = 0; i < 3; i++){
  console.log(i) // 1, 2, 3
}
// console.log(i), Uncaught ReferenceError: gravity is not defined

The scope let and const is the same. The difference is only reassigning. We can not change or reassign the value of const variable. I would strongly suggest you to use let and const, by using let and const you will writ clean code and avoid hard to debug mistakes. As a rule of thumb, you can use let for any value which change, const for any constant value, and for array, object, arrow function and function expression.

📔 Object

Everything can be an object and objects do have properties and properties have values, so an object is a key value pair. The order of the key is not reserved, or there is no order. To create an object literal, we use two curly brackets.

Creating an empty object

An empty object

const person = {}

Creating an objecting with values

Now, the person object has firstName, lastName, age, location, skills and isMarried properties. The value of properties or keys could be a string, number, boolean, an object, null, undefined or a function.

Let us see some examples of object. Each key has a value in the object.

const rectangle = {
  length: 20,
  width: 20
}
console.log(rectangle) // {length: 20, width: 20}

const person = {
  firstName: 'Asabeneh',
  lastName: 'Yetayeh',
  age: 250,
  country: 'Finland',
  city: 'Helsinki',
  skills: [
    'HTML',
    'CSS',
    'JavaScript',
    'React',
    'Node',
    'MongoDB',
    'Python',
    'D3.js'
  ],
  isMarried: true
}
console.log(person)

Getting values from an object

We can access values of object using two methods:

using . followed by key name if the key-name is a one word
using square bracket and a quote

const person = {
  firstName: 'Asabeneh',
  lastName: 'Yetayeh',
  age: 250,
  country: 'Finland',
  city: 'Helsinki',
  skills: [
    'HTML',
    'CSS',
    'JavaScript',
    'React',
    'Node',
    'MongoDB',
    'Python',
    'D3.js'
  ],
  getFullName: function() {
    return `${this.firstName}${this.lastName}`
  },
  'phone number': '+3584545454545'
}

// accessing values using .
console.log(person.firstName)
console.log(person.lastName)
console.log(person.age)
console.log(person.location)

// value can be accessed using square bracket and key name
console.log(person['firstName'])
console.log(person['lastName'])
console.log(person['age'])
console.log(person['age'])
console.log(person['location'])

// for instance to access the phone number we only use the square bracket method
console.log(person['phone number'])

Creating object methods

Now, the person object has getFullName properties. The getFullName is function inside the person object and we call it an object method. The this key word refers to the object itself. We can use the word this to access the values of different properties of the object. We can not use an arrow function as object method because the word this refers to the window inside an arrow function instead of the object itself. Example of object:

const person = {
  firstName: 'Asabeneh',
  lastName: 'Yetayeh',
  age: 250,
  country: 'Finland',
  city: 'Helsinki',
  skills: [
    'HTML',
    'CSS',
    'JavaScript',
    'React',
    'Node',
    'MongoDB',
    'Python',
    'D3.js'
  ],
  getFullName: function() {
    return `${this.firstName} ${this.lastName}`
  }
}

console.log(person.getFullName())
// Asabeneh Yetayeh

Setting new key for an object

An object is a mutable data structure and we can modify the content of an object after it gets created.

Setting a new keys in an object

const person = {
  firstName: 'Asabeneh',
  lastName: 'Yetayeh',
  age: 250,
  country: 'Finland',
  city: 'Helsinki',
  skills: [
    'HTML',
    'CSS',
    'JavaScript',
    'React',
    'Node',
    'MongoDB',
    'Python',
    'D3.js'
  ],
  getFullName: function() {
    return `${this.firstName} ${this.lastName}`
  }
}
person.nationality = 'Ethiopian'
person.country = 'Finland'
person.title = 'teacher'
person.skills.push('Meteor')
person.skills.push('SasS')
person.isMarried = true

person.getPersonInfo = function() {
  let skillsWithoutLastSkill = this.skills
    .splice(0, this.skills.length - 1)
    .join(', ')
  let lastSkill = this.skills.splice(this.skills.length - 1)[0]

  let skills = `${skillsWithoutLastSkill}, and ${lastSkill}`
  let fullName = this.getFullName()
  let statement = `${fullName} is a ${this.title}.\nHe lives in ${this.country}.\nHe teaches ${skills}.`
  return statement
}
console.log(person)
console.log(person.getPersonInfo())

Asabeneh Yetayeh is a teacher.
He lives in Finland.
He teaches HTML, CSS, JavaScript, React, Node, MongoDB, Python, D3.js, Meteor, and SasS.

Object Methods

There are different methods to manipulate an object. Let us see some of the available methods.

Object.assign: To copy an object without modifying the original object

const person = {
  firstName: 'Asabeneh',
  age: 250,
  country: 'Finland',
  city:'Helsinki',
  skills: ['HTML', 'CSS', 'JS'],
  title: 'teacher',
  address: {
    street: 'Heitamienkatu 16',
    pobox: 2002,
    city: 'Helsinki'
  },
  getPersonInfo: function() {
    return `I am ${this.firstName} and I live in ${city}, ${this.country}. I am ${this.age}.`
  }
}

//Object methods: Object.assign, Object.keys, Object.values, Object.entries
//hasOwnProperty

const copyPerson = Object.assign({}, person)
console.log(copyPerson)

Getting object keys using Object.keys()

Object.keys: To get the keys or properties of an object as an array

const keys = Object.keys(copyPerson)
console.log(keys) //['name', 'age', 'country', 'skills', 'address', 'getPersonInfo']
const address = Object.keys(copyPerson.address)
console.log(address) //['street', 'pobox', 'city']

Getting object values using Object.values()

Object.values:To get values of an object as an array

const values = Object.values(copyPerson)
console.log(values)

Getting object keys and values using Object.entries()

Object.entries:To get the keys and values in an array

const entries = Object.entries(copyPerson)
console.log(entries)

Checking properties using hasOwnProperty()

hasOwnProperty: To check if a specific key or property exist in an object

console.log(copyPerson.hasOwnProperty('name'))
console.log(copyPerson.hasOwnProperty('score'))

🌕 You are astonishing. Now, you are super charged with the power of objects. You have just completed day 8 challenges and you are 8 steps a head in to your way to greatness. Now do some exercises for your brain and for your muscle.

💻 Exercises

Exercises: Level 1

Create an empty object called dog
Print the the dog object on the console
Add name, legs, color, age and bark properties for the dog object. The bark property is a method which return woof woof
Get name, legs, color, age and bark value from the dog object
Set new properties the dog object: breed, getDogInfo

Exercises: Level 2

Find the person who has many skills in the users object.

Count logged in users,count users having greater than equal to 50 points from the following object.

const users = {
  Alex: {
    email: 'alex@alex.com',
    skills: ['HTML', 'CSS', 'JavaScript'],
    age: 20,
    isLoggedIn: false,
    points: 30
  },
  Asab: {
    email: 'asab@asab.com',
    skills: ['HTML', 'CSS', 'JavaScript', 'Redux', 'MongoDB', 'Express', 'React', 'Node'],
    age: 25,
    isLoggedIn: false,
    points: 50
  },
  Brook: {
    email: 'daniel@daniel.com',
    skills: ['HTML', 'CSS', 'JavaScript', 'React', 'Redux'],
    age: 30,
    isLoggedIn: true,
    points: 50
  },
  Daniel: {
    email: 'daniel@alex.com',
    skills: ['HTML', 'CSS', 'JavaScript', 'Python'],
    age: 20,
    isLoggedIn: false,
    points: 40
  },
  John: {
    email: 'john@john.com',
    skills: ['HTML', 'CSS', 'JavaScript', 'React', 'Redux', 'Node.js'],
    age: 20,
    isLoggedIn: true,
    points: 50
  },
  Thomas: {
    email: 'thomas@thomas.com',
    skills: ['HTML', 'CSS', 'JavaScript', 'React'],
    age: 20,
    isLoggedIn: false,
    points: 40
  },
  Paul: {
    email: 'paul@paul.com',
    skills: ['HTML', 'CSS', 'JavaScript', 'MongoDB', 'Express', 'React', 'Node'],
    age: 20,
    isLoggedIn: false,
    points: 40
  }
}```

Find people who are MERN stack developer from the users object
Set your name in the users object without modifying the original users object
Get all keys or properties of users object
Get all the values of users object
Use the countries object to print a country name, capital, populations and languages.

Exercises: Level 3

Create an object literal called personAccount. It has firstName, lastName, incomes, expenses properties and it has totalIncome, totalExpense, accountInfo,addIncome, addExpense and accountBalance methods. Incomes is a set of incomes and its description and expenses is a set of incomes and its description.
**** Questions:2, 3 and 4 are based on the following two arrays:users and products ()

    const users = [
    {
        _id: 'ab12ex',
        username: 'Alex',
        email: 'alex@alex.com',
        password: '123123',
        createdAt:'08/01/2020 9:00 AM',
        isLoggedIn: false
    },
    {
        _id: 'fg12cy',
        username: 'Asab',
        email: 'asab@asab.com',
        password: '123456',
        createdAt:'08/01/2020 9:30 AM',
        isLoggedIn: true
    },
    {
        _id: 'zwf8md',
        username: 'Brook',
        email: 'brook@brook.com',
        password: '123111',
        createdAt:'08/01/2020 9:45 AM',
        isLoggedIn: true
    },
    {
        _id: 'eefamr',
        username: 'Martha',
        email: 'martha@martha.com',
        password: '123222',
        createdAt:'08/01/2020 9:50 AM',
        isLoggedIn: false
    },
    {
        _id: 'ghderc',
        username: 'Thomas',
        email: 'thomas@thomas.com',
        password: '123333',
        createdAt:'08/01/2020 10:00 AM',
        isLoggedIn: false
    }
    ];

    const products = [
  {
    _id: 'eedfcf',
    name: 'mobile phone',
    description: 'Huawei Honor',
    price: 200,
    ratings: [
      { userId: 'fg12cy', rate: 5 },
      { userId: 'zwf8md', rate: 4.5 }
    ],
    likes: []
  },
  {
    _id: 'aegfal',
    name: 'Laptop',
    description: 'MacPro: System Darwin',
    price: 2500,
    ratings: [],
    likes: ['fg12cy']
  },
  {
    _id: 'hedfcg',
    name: 'TV',
    description: 'Smart TV:Procaster',
    price: 400,
    ratings: [{ userId: 'fg12cy', rate: 5 }],
    likes: ['fg12cy']
  }
]

Imagine you are getting the above users collection from a MongoDB database. a. Create a function called signUp which allows user to add to the collection. If user exists, inform the user that he has already an account.
b. Create a function called signIn which allows user to sign in to the application

The products array has three elements and each of them has six properties. a. Create a function called rateProduct which rates the product b. Create a function called averageRating which calculate the average rating of a product
Create a function called likeProduct. This function will helps to like to the product if it is not liked and remove like if it was liked.

🎉 CONGRATULATIONS ! 🎉

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