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Declare a variable
Introduction to JavaScript8/17
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  • 1. Why is JavaScript necessary?
  • 2. Introduction to the theme switcher
  • 3. Find an element using querySelector
  • 4. Output the element to the console.
  • 5. Let’s remove the class using classList.remove
  • 6. Let’s add a class to the element using classList.add
  • 7. Declare a variable
  • 8. Introduction to the event handler
  • 9. Let’s make it so that the theme is switched when the button is clicked
  • 10. Introduction to classList.toggle
  • 11. Let’s finish our theme switcher
  • 12. Introduction to the textContent property
  • 13. Change the textual content for the element
  • 14. Uncomment the code
  • 15. We retrieve data from the input field using input.value
  • 16. Let’s use concatenation
  • 17. Summary of “Introduction to JavaScript”
Let’s make it so that the theme is switched when the button is clicked
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Introduction to the event handler

We switched the theme using JavaScript and improved the code by adding a variable. But we were only able to switch the theme on the page once, and we need to make it so that the themes alternate with each other every time we click on the “Change Theme” button.

JavaScript can monitor everything that happens on the page, and we can tell it what to do if the user clicks a button, for example. First, we find the button itself and save it to a variable:

let button = document.querySelector('button');

After the button is found, we tell JavaScript what to do when this button is clicked. This could be, for example, outputting a message to the console:

button.onclick = function() {
  console.log('Click!');
};

Instructions that are executed after the button is clicked can be found inside the curly braces.

From the point of view of the browser, a button click qualifies as an event. The onclick property means “when clicked” and tells JavaScript which event we want to track. And the part of the instruction that is run after onclick is called an event handler. We will analyze this in detail in one of the following chapters.

Although this event is called “click”, it really means any sort of activation — whether by a mouse click, other pointing devices, or with the keyboard.

The “Change Theme” button, which should change themes when it is clicked, has the theme-button class. Let’s find it and see how the event handler works. Let’s use the console again to test it: tell JavaScript to output the message each time the button is clicked.

JavaScript has another method for creating an event handler. We will discuss it in the following chapters, but if you’re curious, you can read about it in the JavaScript documentation.

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  • index.html
  • style.css
  • script.js
HTML
<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <link rel="stylesheet" href="setting.css"> <link rel="stylesheet" href="style.css"> <title>FlashNews!</title> </head> <body class="page light-theme"> <header class="page-header"> <div class="container"> <a class="header-link" href="#">Main page</a> <button class="theme-button" type="button">Change the theme</button> </div> </header> <main class="inner-main"> <div class="container"> <h1 class="inner-heading">Subscribe to our newsletter</h1> <p class="subscription-message">We promise to send you news no more than once a day. We will only send you the most interesting and important stories.</p> <form action="https://echo.htmlacademy.ru/courses" method="post" class="subscription"> <div class="subscription-inner"> <label class="subscription-label" for="subscription-email">email</label> <input type="email" class="subscription-email" placeholder="muffin@flashnews.ru" value="" required id="subscription-email"> </div> <button class="button subscription-button" type="submit">Subscribe</button> </form> <section class="news-list list-tiles-view"> <h2 class="news-list-heading">Or continue reading more news stories.</h2> <article class="new-block"> <img src="img/new-graph.jpg" alt="New library"> <div class="new-block-text"> <h3>New graphics library</h3> <p>Now you can create a dashboard in just a matter of seconds.</p> <time datetime="2019-10-16">October 16, 2019</time> </div> </article> <article class="new-block"> <img src="img/new-robot.jpg" alt="What is going on with robots?"> <div class="new-block-text"> <h3>What is going on with robots?</h3> <p>A lot of interesting things are happening in robotics. This could be a piece of news about the latest developments, but that is not what this is.</p> <time datetime="2019-10-15">October 15, 2019</time> </div> </article> </section> </div> </main> <footer class="page-footer"> <div class="container"> <p>© FlashNews!</p> <a class="footer-logo"> <img src="img/white-logo.svg" alt="FlashNews! portal logo"> </a> </div> </footer> <script src="script.js"></script> </body> </html>
CSS
/* Light theme */ .light-theme { color: #333333; background-color: #eae9f2; } .light-theme .page-header { background-color: #ffffff; box-shadow: 0 2px 2px rgba(0, 0, 0, 0.2); } .light-theme .header-link { color: #6653d9; } .light-theme .header-link::before { background: url("img/arrow-back-light.svg") no-repeat 0 0; } .light-theme .theme-button { color: #6653d9; border: 1px solid #6653d9; } .light-theme .theme-button::before { background-image: url("img/moon-normal.svg"); } .light-theme .theme-button:focus, .light-theme .theme-button:hover { color: #ffffff; background-color: #6653d9; } .light-theme .theme-button:focus::before, .light-theme .theme-button:hover::before { background-image: url("img/moon-hover.svg"); } .light-theme .menu-open { background-color: #ffffff; background-image: url("img/menu-open-light.svg"); } .light-theme .menu-open:focus, .light-theme .menu-open:hover { background-color: #6653d9; background-image: url("img/menu-open-dark.svg"); } .light-theme .menu-close { background-color: #6653d9; } .light-theme .menu-close:focus, .light-theme .menu-close:hover { background-color: #473c8d; } .light-theme .main-menu { background-color: #6653d9; color: #ffffff; } .light-theme .main-menu a:focus, .light-theme .main-menu a:hover { background-color: #473c8d; } .light-theme .news-view button { border: 1px solid #6653d9; color: #6653d9; } .light-theme .news-view button:focus, .light-theme .news-view button:hover, .light-theme .news-view button:active, .light-theme .news-view .view-checked { background-color: #6653d9; color: #ffffff; } .light-theme .news-view .row-view:focus::before, .light-theme .news-view .row-view:hover::before, .light-theme .news-view .row-view:active::before { background-image: url("img/rows-light-checked.svg"); } .light-theme .news-view .tile-view:focus::before, .light-theme .news-view .tile-view:hover::before, .light-theme .news-view .tile-view:active::before { background-image: url("img/tiles-light-checked.svg"); } .light-theme .row-view::before { background-image: url("img/rows-light.svg"); } .light-theme .tile-view::before { background-image: url("img/tiles-light.svg"); } .light-theme .row-view::before { background-image: url("img/rows-light.svg"); } .light-theme .row-view.view-checked::before { background-image: url("img/rows-light-checked.svg"); } .light-theme .tile-view.view-checked::before { background-image: url("img/tiles-light-checked.svg"); } .light-theme .new-block { background-color: #ffffff; box-shadow: 0 1px 4px rgba(0, 0, 0, 0.2); } .light-theme .new-block time { color: #aaaaaa; } .light-theme .cookies-agreement { background-color: #fdeacd; box-shadow: 0 -1px 4px rgba(0, 0, 0, 0.2); } .light-theme .button { background-color: #6653d9; color: #ffffff; } .light-theme .button:focus, .light-theme .button:hover { background-color: #473c8d; } .light-theme .page-footer { background-color: #6653d9; color: #ffffff; } .light-theme .subscription { background-color: #ffffff; box-shadow: 0 1px 4px rgba(0, 0, 0, 0.2); } .light-theme .subscription-message { background-color: #6653d9; color: #eae9f2; } .light-theme .subscription-message::before { background-image: url("img/icon-ok-light.svg"); } .light-theme .subscription-label { color: #6653d9; } .light-theme .subscription-email { border-bottom: 1px solid #6653d9; color: #333333; } /* Dark theme */ .dark-theme { color: #f2f2f2; background-color: #17161a; } .dark-theme .page-header { background-color: #373540; box-shadow: 0 2px 2px rgba(0, 0, 0, 0.6); } .dark-theme .header-link { color: #9484f2; } .dark-theme .header-link::before { background: url("img/arrow-back-dark.svg") no-repeat 0 0; } .dark-theme .theme-button { color: #9484f2; border: 1px solid #9484f2; } .dark-theme .theme-button::before { background-image: url("img/sun-normal.svg"); } .dark-theme .theme-button:focus, .dark-theme .theme-button:hover { color: #17161a; background-color: #9484f2; } .dark-theme .theme-button:focus::before, .dark-theme .theme-button:hover::before { background-image: url("img/sun-hover.svg"); } .dark-theme .menu-open { background-color: #373540; background-image: url("img/menu-open-dark.svg"); } .dark-theme .menu-open:focus, .dark-theme .menu-open:hover { background-color: #473c8d; background-image: url("img/menu-open-dark.svg"); } .dark-theme .menu-close { background-color: #473c8d; } .dark-theme .menu-close:focus, .dark-theme .menu-close:hover { background-color: #6653d9; } .dark-theme .main-menu { background-color: #473c8d; color: #f2f2f2; } .dark-theme .main-menu a:focus, .dark-theme .main-menu a:hover { background-color: #6653d9; } .dark-theme .news-view button { border: 1px solid #9484f2; color: #9484f2; } .dark-theme .news-view button:focus, .dark-theme .news-view button:hover, .dark-theme .news-view button:active, .dark-theme .news-view .view-checked { background-color: #9484f2; color: #17161a; } .dark-theme .news-view .row-view:focus::before, .dark-theme .news-view .row-view:hover::before, .dark-theme .news-view .row-view:active::before { background-image: url("img/rows-dark-checked.svg"); } .dark-theme .news-view .tile-view:focus::before, .dark-theme .news-view .tile-view:hover::before, .dark-theme .news-view .tile-view:active::before { background-image: url("img/tiles-dark-checked.svg"); } .dark-theme .row-view::before { background-image: url("img/rows-dark.svg"); } .dark-theme .tile-view::before { background-image: url("img/tiles-dark.svg"); } .dark-theme .row-view.view-checked::before { background-image: url("img/rows-dark-checked.svg"); } .dark-theme .tile-view.view-checked::before { background-image: url("img/tiles-dark-checked.svg"); } .dark-theme .new-block { background-color: #2a2930; box-shadow: 0 1px 4px rgba(0, 0, 0, 0.6); } .dark-theme .new-block time { color: #888888; } .dark-theme .cookies-agreement { background-color: #473c8d; box-shadow: 0 -1px 4px rgba(0, 0, 0, 0.6); } .dark-theme .button { background-color: #9484f2; color: #17161a; } .dark-theme .button:focus, .dark-theme .button:hover { background-color: #b6aaff; } .dark-theme .page-footer { background-color: #0a0910; color: #f2f2f2; } .dark-theme .subscription { background-color: #2a2930; box-shadow: 0 1px 4px rgba(0, 0, 0, 0.6); } .dark-theme .subscription-message { background-color: #9484f2; color: #17161a; } .dark-theme .subscription-message::before { background-image: url("img/icon-ok-dark.svg"); } .dark-theme .subscription-label { color: #9484f2; } .dark-theme .subscription-email { border-bottom: 1px solid #9484f2; color: #f2f2f2; }
JavaScript
let page = document.querySelector('.page'); page.classList.remove('light-theme'); page.classList.add('dark-theme');

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    1. On line 5, declare a variable themeButton and write an element to it with the class theme-button.
    2. On the next line, add the following event handler: themeButton.onclick = function() {};.
    3. Inside the curly braces of the handler, add the following to be output to the console: console.log('Button pressed!');.
    4. Double click on the “Change Theme” button in the mini-browser
    5. Verify that the two new messages appear in the console.

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