day 1

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 draft.md
 playground
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--- a/01-Day/helloworld.js
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 console.log('Hello, World!')
--- a/01-Day/index.html
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 <!DOCTYPE html>
 <html>
 <head>
    <title>30DaysOfJavaScript</title>
 </head>
 <body>
        <button onclick="alert('Welcome to 30DaysOfJavaScript!');">Click Me</button>
        <script src="./helloworld.js"></script>
        <script src="./introduction.js"></script>
        <script src="./varaible.js"></script>
        <script src="./main.js"></script>
 </body>
 </html>
--- a/01-Day/introduction.js
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 console.log('Welcome to 30DaysOfJavaScript')
--- a/01-Day/main.js
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 console.log(firstName, lastName, country, city, age, isMarried) 
 console.log(gravity, boilingPoint, PI) // 9.81, 100, 3.14
 console.log(name, job, live)
--- a/01-Day/varaible.js
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 // Declaring different variables of different data types
 let firstName = 'Asabeneh' // first name of a person
 let lastName = 'Yetayeh' // last name of a person
 let country = 'Finland' // country
 let city = 'Helsinki' // capital city
 let age = 100 // age in years
 let isMarried = true
 // Declaring variables with number values
 const gravity = 9.81 // earth gravity  in m/s2
 const boilingPoint = 100 // water boiling point, temperature in oC
 const PI = 3.14 // geometrical constant
 // Variables can also be declaring in one line separated by comma
 let name = 'Asabeneh', //name of a person
  job = 'teacher',
  live = 'Finland'
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 ##  Table of Contents
 ![Thirty Days Of JavaScript](./images/30DaysOfJavaScript.png)
 - [📔 Day 1](#%f0%9f%93%94-day-1)
 - [Introduction](#introduction)
 - [Requirements](#requirements)
 - [Setup](#setup)
  - [Install Node.js](#install-nodejs)
  - [Browser](#browser)
    - [Installing Google Chrome](#installing-google-chrome)
    - [Opening Google Chrome Console](#opening-google-chrome-console)
    - [Writing Code on browser Console](#writing-code-on-browser-console)
      - [Console.log](#consolelog)
      - [Console.log with multiple arguments](#consolelog-with-multiple-arguments)
      - [Comment](#comment)
      - [Syntax](#syntax)
      - [Arithmetics](#arithmetics)
  - [Code Editor](#code-editor)
    - [Installing Visual Studio Code](#installing-visual-studio-code)
    - [How to use visual studio code](#how-to-use-visual-studio-code)
 - [Adding JavaScript to a web page](#adding-javascript-to-a-web-page)
  - [Inline Script](#inline-script)
  - [Internal script](#internal-script)
  - [External script](#external-script)
  - [Multiple External scripts](#multiple-external-scripts)
 - [Introduction to Data types](#introduction-to-data-types)
  - [Number](#number)
  - [String](#string)
  - [Booleans](#booleans)
  - [Undefined](#undefined)
  - [Null](#null)
  - [Checking Data types](#checking-data-types)
 - [Comments](#comments)
 - [Variables](#variables)
 - [💻 Day 1: Exercises](#%f0%9f%92%bb-day-1-exercises)
 # 📔 Day 1
 # Introduction
 **Congratulations** for deciding to participate in a 30 days of JavaScript programming challenge . In this challenge you will learn everything you need to be a JavaScript programmer and in general the whole concepts of programming. In the end of the challenge you will get a 30DaysOfPython programming challenge certificate. Join the [telegram group](https://t.me/ThirtyDaysOfJavaScript).
 **A 30DaysOfJavaScript** is a guide for both beginners and advanced JavaScript developers. Welcome to JavaScript. JavaScript is the language of the browser.
 In this step by step tutorial, you will learn JavaScript, the most popular programming language in the history of mankind.
 You use JavaScript **_to add interactivity to websites, to develop mobile apps, desktop applications, games_** and nowadays JavaScript can be used for **_machine learning_** and **_AI_**.
 **_JavaScript (JS)_** has increased in popularity in recent years and has been the leading
 programming language for four consecutive years and is the most used programming language on
 Github.
 # Requirements
 No prior knowledge of programming is required to follow this challenge. You need only:
 1. Motivation
 2. Computer
 3. Internet
 4. Browser
 5. Code Editor
 # Setup
 I believe you have the motivation, computer and internet. 
 ## Install Node.js
 You may not need it right now but you may need it for later. Install [node.js](https://nodejs.org/en/).
 ![Node download](images/download_node.png)
 After downloading double click and install
 ![Install node](images/install_node.png)
 We can check if node is installed in our machine by opening our device terminal or command  prompt.
 ```sh
 asabeneh $ node -v
 v12.14.0
 ```
 I am using node version 12.14.0 which is the recommended version of node.
 ## Browser
 There are many browsers out there but I strongly recommend Google Chrome.
 ### Installing Google Chrome
 Install [google chrome](https://www.google.com/chrome/) if you do not have one yet. We can write small JavaScript code on the browser console but we can not use the browser console to develop applications.
 ### Opening Google Chrome Console
 You can open the google chrome either by clicking three dots at the top right corner of chrome or using short cut. I prefer short cuts.
 ![Opening chrome](images/opening_developer_tool.png)
 To open chrome console using short cut. 
 ```sh
 Mac
 Command+Option+I
 Windows:
 Ctl+Shift+I
 ```
 ![Opening console](images/opening_chrome_console_shortcut.png)
 After you open the google chrome console try to explore the marked buttons. We will spend most of the time on the Console part. The Console is the place where your JavaScript code goes. The google console V8 engine change your JavaScript code to machine code.
 Let us write a JavaScript code on google chrome console:
 ![write code on console](./images/js_code_on_chrome_console.png)
 ### Writing Code on browser Console
 We can write any JavaScript code on google console or any browser console but for this challenge we only focus on google chrome console. Open the console using:
 ```sh
 Mac
 Command+Option+I
 Windows:
 Ctl+Shift+I
 ```
 #### Console.log
 To write our first JavaScript code we used a builtin function **console.log()**. We passed an argument as an input data and the function display the output. We passed 'Hello, World' as input data or argument in the condole.log() function.
 ```js
 console.log('Hello, World!')
 ```
 #### Console.log with multiple arguments
 The console.log(param1, param2, param3), can take multiple arguments.
 ![console log multiple arguments](./images/console_log_multipl_arguments.png)
 ```js
 console.log('Hello', 'World', '!')
 console.log('HAPPY', 'NEW', 'YEAR', 2020)
 console.log('Welcome', 'to', 30, 'Days', 'Of', 'JavaScript')
 ```
 As you can see from the above snippet code, *console.log()* can take multiple arguments.
 Congratulations! You wrote your first JavaScript code using *console.log()*. 
 #### Comment
 We add comment  to our code. Comment is very important to make code more readable and to leave remark in our code. JavaScript does not execute comment part of our code. Any text starts with // in JavaScript is a comment or any thing enclose like this /*  */ is a comment.
 Example: Single Line Comment
    // This is the first comment
    // This is the second comment
    // I am a single line comment
 Example: Multiline Comment
  /*
  This is a multiline comment
  multiline comment take multiple lines.
  JavaScript is the language the langauge of the web.
  */
 #### Syntax
 JavaScript is a programming language and it has its own syntax like other languages. If we do not write a syntax which JavaScript understands it will raise different kind of errors. We will see different kind of JavaScript errors later but for now let us see syntax error.
 ![Error](images/raising_syntax_error.png)
 I made a deliberate mistake the console raise a syntax error. Actually, the syntax is very informative. It tell what kind of mistake we made and we can fix the by reading error feedback.  The process of identifying and removing errors from a program is called debugging. Let us fix the errors:
 ```js
 console.log("Hello, World!")
 console.log('Hello, World!')
 ```
 So far, we so how to display text using a *console.log()*. If we are printing text or string using *console.log()*, the text has to be under single, double or back tick.
 **Example:**
 ```js
 console.log("Hello, World!")
 console.log('Hello, World!')
 console.log(`Hello, World!`)
 ``` 
 #### Arithmetics
 Now, let us practice  more writing JavaScript code using *console.log()* on google chrome console for number data types. 
 In addition to text, we can also do mathematical calculations using JavaScript. Let us do the following simple calculations
 ![Arithmetic](images/arithmetic.png)
 ```js
 console.log(2 + 3)   // Addition
 console.log(3 - 2)   // Subtraction
 console.log(2 * 3)   // Multiplication  
 console.log(3 / 2)   // Division
 console.log(3 % 2)   // Modulus - finding remainder 
 console.log(3 ** 2)  // Exponential
 ``` 
 ## Code Editor
 We can write code on the browser console but it won't be for a big project. In real work environment, developers use different code editors to write codes. In this 30 days python programming challenge we will use visual studio code. 
 ### Installing Visual Studio Code
 Visual studio code is a very popular open source text editor and I would recommend to [download](https://code.visualstudio.com/) visual studio code, but if you are in favor of other editors, feel free to follow with what you have.
 ![Vscode](images/vscode.png) 
 If you installed visual studio code, let us start using it.
 ### How to use visual studio code
 Open the visual studio code by double clicking the visual studio icon. When you open it you will get this kind of interface. Try to interact with the labelled icons.
 ![Vscode ui](./images/vscode_ui.png)
 ![Vscode add project](./images/adding_project_to_vscode.png)
 ![Vscode open project](./images/opening_project_on_vscode.png)
 ![script file](images/scripts_on_vscode.png)
 ![running script](./images/running_script.png)
 ![coding running](./images/launched_on_new_tab.png)
 # Adding JavaScript to a web page
 JavaScript can be added to a web page in three ways:
 - **_Inline script_**
 - **_Internal script_**
 - **_External script_**
 - **_Multiple External scripts_**
 The following sections show different ways of adding JavaScript code to your web page.
 ## Inline Script
 Create a folder on your desktop and  call it 30DaysOfJS or in any location and create an **_index.html_** file in project folder. Then paste the following code and open it in a browser, either in [Chrome](https://www.google.com/chrome/).
 ```html
 <!DOCTYPE html>
  <html>
    <head>
      <title>30DaysOfScript:Inline Script</title>
    </head>
    <body>
      <button onclick="alert('Welcome to 30DaysOfJavaScript!');">Click Me</button>
    </body>
  </html>
 ```
 Now, you wrote your first inline script. We can create a pop up alert message using the build in *alert()* function.
 ## Internal script
 Internal script can be written in the _head_ or the _body_ but it is preferred to put it on the body of the html document.
 First let us write on the head part of the page.
 ```html
 <!DOCTYPE html>
  <html>
    <head>
       <title>30DaysOfScript:Internal Script</title>
      <script>
        console.log("Welcome to 30DaysOfJavaScript");
      </script>
    </head>
    <body>
    </body>
  </html>
 ```
 This is how we write inline script most of the time. Writing the JavaScript code in the body section is the most preferred place. Open the browser console to see the out put from the console.log()
 ```html
 <!DOCTYPE html>
  <html>
    <head>
      <title>30DaysOfScript:Internal Script</title>
    </head>
    <body>
       <button onclick="alert('Welcome to 30DaysOfJavaScript!');">Click Me</button>
      <script>
        console.log("Welcome to 30DaysOfJavaScript");
      </script>
    </body>
  </html>
 ```
 Open the browser console to see the out put from the console.log()
 ![js code from vscode](./imas/js_code_ve)
 ## External script
 Similar to the internal script, the external script link can be on the header or body but it is preferred to put it in the body.
 First we should create an external JavaScript file with .js extension. Any JavaScript file ends with .js. Create a file introduction.js inside your project directory and write the following code and link this fat the body
 ```js
 console.log('Welcome to 30DaysOfJavaScript')
 ```
 External script in the head
 ```html
 <!DOCTYPE html>
  <html>
    <head>
      <title>30DaysOfJavaScript:External script</title>
      <script src="introduction.js"></script>
    </head>
    <body>
    </body>
    </html
 ```
 External Script in the body
 ```html
 <!DOCTYPE html>
  <html>
    <head>
      <title>30DaysOfJavaScript:External script</title>
    </head>
    <body>
      //it could be in the header or in the body 
      // Here is the recommended place to put the external script
      <script src="introduction.js"></script>
    </body>
    </html
 ```
 Open the browser console to see the out put from the console.log()
 ## Multiple External scripts
 We can link multiple external JavaScript files to a web page.
 Create  helloword.js file inside 30DaysOfJS folder and write the following code
 ```js
 console.log('Hello, World!')
 ```
 ```html
 <!DOCTYPE html>
  <html>
    <head>
      <title>Multiple External Scripts</title>
    </head>
    <body>
      <script src ="./helloworld.js"></script>
      <script src="./introduction.js"></script>
    </body
    </html
 ```
 You main.js file should be below all other scripts. Watch out your exercise needs to understand this line.
 ![Multiple Script](./images/multiple_script.png)
 # Introduction to Data types
 In JavaScript and also other programming languages there are different kinds of data types. The following are JavaScript primitive data types:_String, Number, Boolean, undefined, Null_ and _Symbol_.
 ## Number
    - Integer: Integer(negative, zero and positive) numbers
        Example:
        ... -3, -2, -1, 0, 1, 2, 3 ...
    - Float: Decimal number
        Example
        ... -3.5, -2.25, -1.0, 0.0, 1.1, 2.2, 3.5 ...
 ## String
 A collection of one or more characters under a single quote , double quote or back stick.
 **Example:**
 ```js
 'Asabeneh'
 'Finland'
 'JavaScript is a beautiful programming language'
 "I love teaching"
 'I hope you are enjoying the first day'
 `We can also create a string using a back tick`
 ```
 ## Booleans
 Boolean value is either true or false. Any comparisons return a boolean value which is either true or false.
 A boolean data type is either a True or False value.
 **Example:**
 ```js
    true  #  if the light on ,the value is true
    false # if the light off, the value is False
 ```
 ## Undefined
 In JavaScript if we don't assign a value to a variable the value is undefined. In addition to that if a function is not returning anything it returns undefined.
 ```js
 let firstName;
 console.log(firstName); //not defined, because it is not assigned to a value yet
 ```
 ## Null
 Null in JavaScript means an empty value.
 ## Checking Data types
 To check the data type of a certain data type we use the **typeOf** operator. See the following example.
 ```js
 console.log(typeof 'Asabeneh') // string
 console.log(typeof 5)          // number
 console.log(typeof true )     // boolean
 console.log(typeof null)      // object type
 console.log(typeof undefined)  // undefined
 ```
 # Comments
 Commenting in JavaScript is similar to other programming languages. Comments can help to make code more readable.
 There are two ways of commenting:
 - _Single line commenting_
 - _Multiline commenting_
 ```js
 // let firstName = 'Asabeneh'; single line comment
 // let lastName = 'Yetayeh'; single line comment
 ```
 Multiline commenting:
 ```js
 /*
    let location = 'Helsinki';
    let age = 100;
    let isMarried = true;
    This is a Multiple line comment
    */
 ```
 # Variables
 Variables are _containers_ of data. Variables _store_ data in a memory location. When a variable is declared a memory location is reserved and when it is assigned to a value, the memory space will be filled with that data. To declare a variable we use, _var_, _let_ or _const_ key words. I will talk about var, let and const in detail in other section(scope). For now, the above explanation is enough.
 For a variable which changes at different time we use _let_ but if the data doesn't change at all we use _const_. For example PI, country name, gravity do no change and we can use *const*.
 A JavaScript variable name  shouldn't begin with a number
 A JavaScript variable name does not allow special characters except dollar sign and underscore. 
 A JavaScript variable name follow a camelCase convention.
 A JavaScript variable name shouldn't have space between words. The following are valid examples of JavaScript variables.
 Valid variables in JavaScript:
 ```js
    firstName
    lastName
    country
    city
    capitalCity
    age
    isMarried
    first_name
    last_name
    is_marreid
    capital_city
    num1
    num_1
    _num_1
    $num1
    year2019
    year_2019
 ```
 Camel case or the first way of declaring is conventional in JavaScript. In this material, camelCase variables will be used.
 Invalid variable:
 ```js
  first-name
  1_num
  num_#_1
 ```
 ```js
 // Declaring different variables of different data types
 let firstName = 'Asabeneh'; // first name of a person
 let lastName = 'Yetayeh'; // last name of a person
 let country = 'Finland'; // country
 let city = 'Helsinki'; // capital city
 let age = 100; // age in years
 let isMarried = true;
 console.log(firstName, lastName, country, city, age, isMarried); //Asabeneh, Yetayeh, Finland, Helsinki, 100, True
 // Declaring variables with number values
 const gravity = 9.81; // earth gravity  in m/s2
 const boilingPoint = 100; // water boiling point, temperature in oC
 const PI = 3.14; // geometrical constant
 console.log(gravity, boilingPoint, PI); // 9.81, 100, 3.14
 // Variables can also be declaring in one line separated by comma
 let name = 'Asabeneh', //name of a person
  job = 'teacher',
  live = 'Finland';
 console.log(name, job, live);
 ```
 When you run the files on 01-Day folder you should get this:
 ![Day one](./images/day-1.png)
 # 💻 Day 1: Exercises
 1. Write a single line comment which says, _comments can make code readable_
 2. Write another single comment which says, *welcome to 30DaysOfJavaScript*
 3. Write a multiline comment which says, _comments can make code readable, easy to use_
   _and informative_
 4. Create a varaible.js file and declare variables and assign string, boolean, undefined and null data types
 5. Create datatypes.js file and use the JavaScript ***typeof*** operator to check different data types. Check the data type of each variables
 6. Declare four variables without assigning values
 7. Declare four variables with assigning values
 8. Declare variables to store your first name, last name, marital status, country and age in multiple lines
 9. Declare variables to store your first name, last name, marital status, country and age in a single line
 10. Declare two variables _myAge_ and _yourAge_ and assign them initial values and log to browser console.
   Output:
   ```sh
   I am 25 years old.
   You are 30 years old.
   ```
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							`console.log('Welcome to 30DaysOfJavaScript')`