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HTML

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HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

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Adding Classes

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. So you don't have access to higher-up elements like the <html> tag. If you want to add classes there that can affect the whole document, this is the place to do it.

About the <head>

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. If you need things in the <head> of the document, put that code here.

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CSS

About CSS Preprocessors

CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

About Vendor Prefixing

To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance -webkit- or -moz-.

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JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

Add External Scripts/Pens

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Packages

Add Packages

Search for and use JavaScript packages from npm here. By selecting a package, an import statement will be added to the top of the JavaScript editor for this package.

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Using packages here is powered by Skypack, which makes packages from npm not only available on a CDN, but prepares them for native JavaScript ES6 import usage.

All packages are different, so refer to their docs for how they work.

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HTML 

              
                <div class="container">
  <div class="row no-gutters"id="header-row">
    <div class="col-sm-2">
       <h2>You used to call me....</h2>
    </div> <!-- end of col-sm-4 -->
    <div class="col-sm-8">
      <img src="https://gph.is/2JKqoxO" id="header-pic">
    </div> <!-- end of col- sm- 4 -->
        <div class="col-sm-2">
       <h2>Callback functions in JavaScript</h2>
    </div> <!-- end of col-sm-4 -->
  </div><!-- end of row -->

  <div class="row">
    <div class="col-sm-6">
      <h2>Background</h2>
      <p>JS is considered to be synchronous i.e. single threaded. So one line of code is executed at a time. It cannot multi-task and move on to the next line/block of code till the current line/block has been executed.</p>
      <p>So if you have a JavaScript function which has to perform some complex computations, it may well be possible that during that time other parts of the application freeze..</p>
        <img src="data:image/jpeg;base64,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">
      <p>If you're reading this then you are probably acquainted with the idea that almost everything in JS is an object (primitive types behave as objects as they have an object wrapper class around them). This includes functions.</p>
      <p>So functions are objects i.e the typeof is object,which means:
        <ul>
   <li>Functions can be stored in variables</li>
<li>You can pass in a function as an argument to another function</li>
  <li>You can return a function, from a function</li>
 <li>One function can be created inside another</li>
          <p><i>These are aspects of what is known as asychronous programming</i></p>
      </ul>
      </p>
    <p><b>Some use cases of callback functions</b></p>
    <ul>
      <li>DOM events sucha s clicks and User Input/Output</li>
      <li>Databases (e.g. MySQL,MongoDB,CouchDB etc)</li>
      <li> Fetching data from APIs (e.g. Facebook, Twitter, Push Notifications)</li>
      <li>HTTP and web socket connections</li>      
    </ul>
    
    </div> <!-- end of col-sm-6 -->
    <div class="col-sm-6">
      <h2>Callback functions</h2>
      <p><mark>A function that is passed as an argument to another function to be called, is called a Callback function</mark></p>
      <p>The function that accepts another function as an argument is called a higher-order function and contains the logic for when the callback function is executed. This outer function undertakes the async task.</p>
      <p><q>A reference to executable code, or a piece of executable code, that is passed as an argument to other code. - Wikipedia</q></p>
      <p>But why?</p>
      <p><mark>Callback functions are efficient as a function doesn't have to wait for something to finish/return a value in order to complete a routine/task</mark>These are often used in DOM events.</p>
      <p></p>
      <p><i>Callback functions can be asynchronous or synchronous.
</i></p>
      <p>In a normal function some value is returned and stored in a variable to presumably perform some other task. However, with callback functions the callback funtion is passed as an argument to the main function and, the value returned from the main function is provided instead to the callback function.</p>
      <p>It is also a way of making sure that an event  for example "click" has taken place before something else runs.</p>
      <p>So the callback function is available but won't run till an event for example "click" event occurs</p>
          <p>Example:</p>
        <code>
          <span class="comment">//mainFunc accepts a callback function as a parameter</span><br>
    function mainFunc(item1, item2, callback) {<br>
  alert('I like eating: ' + item1 + 'and ' + item2);<br>
 <span class="comment">//callback function is called after the above lines have finished executing</span><br>       
  callback();<br>
}<br>
mainFunc('chocolate', 'cheese', function() {<br>
  alert('Do you have some?');<br>
});<br>
           </code>
</div> <!-- end of col-sm-6 -->
  </div> <!-- end of row -->
  <div class='row'>
      <div class="col-sm-6">
      <h2>What's happening?</h2>
        <p>When we pass a function as an argument to another function, we are treating it like a variable. </p>
        <p>Only the function defination is passed. The function is not executed in the parameter.</p>
        <p>The Callback function is executed at some point later within the original function's body.</p>
        <p> JavaScript is still executing the code/functions in the order given. The only change is that JS doesn't wait for a response before moving on.</p>
</div> <!-- end of col-sm-6 -->
      <div class="col-sm-6">
      <h2>How</h2>
 <ol>
<li><p>Pass a named function to be called immediately after the main function. Named functions don't have '( )' when passed as a function parameter to another function</p>
   </li>
   <li><p>Anonymous function as a parameter to another function </p>
   </li>
    <li> You can store a reference to the callback function in a named variable </li>
    </ol>
</div> <!-- end of col-sm-6 -->
</div> <!-- end of row -->
<div class="row">
  <div class="col-md-6">
    <h2>Exercise</h2>
    <p>1. Code a callback function which runs upon a click event on the button below to alert the string "hello"</p>
    <p>2. Code a callback function that will alert the string "bye" 5 seconds after the callback function following the click event</p>
    <div class='row'>
      <div class="col-sm-6">
<img src ="https://media.giphy.com/media/l378hTn2fvdjSNhew/giphy.gif">
      </div> <!-- end of nested col-6 -->
      <div class="col-sm-6">
        <p> This is the button to work with. It has an id of "myButton" via which you can select it in  your JS file or here on codepen </p>
        <p><button class="btn btn-success btn-lg" id="myButton">Button</button></p>
        <p><i class="fa fa-arrow-down fa-3x"></i></p>
        <p>Code a function called which is called after the click event. The function  uses an anonymous function as a callback and alerts ("This is alerted 2nd").
        <p><i class="fa fa-arrow-down fa-3x"></i></p>
        <p>Code a function called <mark>"callBackFunction"</mark> which is  referenced by the variable <code>var callbackFuncVar</code> and is called after the click event and alerts("bye")</p>
        <p>Call the callback function referenced by var callbackFuncVar within this main function</p></p>
      </div> <!-- end of nested col-6 -->
    </div> <!-- end of nested row -->
  </div><!-- end of col-sm-6 -->
    <div class="col-md-6">
      <h2>Examples:</h2>
      <ol>
        <li> <p>Named Callback function</p>
          <code><button id="myId">click</button><br>
          var button = document.getElementById("myButton");
          button.addEventListener("click");
          button.addEventListener("click", hello());
          <br>
          function hello(){ <br>
          alert("hello"); <br>
          }       
</code>
       </li> 
        <li> <p>AnonymousCallback function</p><code>
          button.addEventListener("click", function(){<br>
           alert("hello");<br>
          });        
          </code></li>
        <li> <p>Using a named variable</p><code>
          function addNum(num1, num2 , callback){ </br>
   heading.innerHTML+= ( "This is displayed first" + num1 + num2); <br>
   callback(); <br>
      } </br>
     
      <span class="comment"> // display() function is called just after the ending of addNum()</span> <br>
   var display = function display(){ <br>
    heading.innerHTML+= ("This is printed after addition"); <br>
   } <br>
     
      <span class="comment">// Calling addNum()</span><br>
      function 
   addNum(1,6,display);          
          </code>
        </li>
      </ol>
  </div><!-- end of col-sm-6 -->
</div><!-- end of row -->
    <div class="row">
    <div class="col-sm-4">
      <blockquote class="twitter-tweet" data-lang="en"><p lang="en" dir="ltr">JavaScript defines a `callback` differently than other languages. Callbacks typically happens afterwards. In JavaScript this is not the case. callback can refer to a function pointer and be synchronous. I thought I saw this from <a href="https://twitter.com/rauschma?ref_src=twsrc%5Etfw">@rauschma</a>, but I can&#39;t find it now.</p>&mdash; JavaScript Joel 👾 (@joelnet) <a href="https://twitter.com/joelnet/status/1120792338165420032?ref_src=twsrc%5Etfw">April 23, 2019</a></blockquote>
<script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>
    </div><!-- end of col-sm-4 -->
    <div class="col-sm-4">
    <blockquote class="twitter-tweet" data-lang="en"><p lang="en" dir="ltr">Day 6 <a href="https://twitter.com/hashtag/100DaysOfCode?src=hash&amp;ref_src=twsrc%5Etfw">#100DaysOfCode</a> Forgot to tweet yesterday. <br><br>Learned about the DOM (Document Object Model) and callback functions. A callback function is a function that we pass into another function as an argument, which will then call the function for us. <a href="https://twitter.com/hashtag/javascript?src=hash&amp;ref_src=twsrc%5Etfw">#javascript</a> <a href="https://twitter.com/hashtag/ES6?src=hash&amp;ref_src=twsrc%5Etfw">#ES6</a></p>&mdash; Ian (@iandv6) <a href="https://twitter.com/iandv6/status/1104976471402065921?ref_src=twsrc%5Etfw">March 11, 2019</a></blockquote>
<script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>
      <blockquote class="twitter-tweet" data-lang="en"><p lang="en" dir="ltr">My answer to How does the Javascript callback function work? <a href="https://t.co/KzGci2jf34">https://t.co/KzGci2jf34</a></p>&mdash; Neha Sharma (@hellonehha) <a href="https://twitter.com/hellonehha/status/1118450069177602048?ref_src=twsrc%5Etfw">April 17, 2019</a></blockquote>
<script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>

  </div>
        <div class="col-sm-4">
          <blockquote class="twitter-tweet" data-lang="en"><p lang="en" dir="ltr">Javascript Callback Function Example <a href="https://t.co/BwlbTsjYhl">https://t.co/BwlbTsjYhl</a></p>&mdash; Web Code Geeks (@webcodegeeks) <a href="https://twitter.com/webcodegeeks/status/1121428678888579075?ref_src=twsrc%5Etfw">April 25, 2019</a></blockquote>
<script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>
          <blockquote class="twitter-tweet" data-lang="en"><p lang="en" dir="ltr">Two JavaScript functions on the phone:<br><br>— &quot;Don&#39;t worry, I&#39;ll give you a callback.&quot;<br>— &quot;Promise?&quot;</p>&mdash; Eduardo Bouças (@eduardoboucas) <a href="https://twitter.com/eduardoboucas/status/627807319687081984?ref_src=twsrc%5Etfw">August 2, 2015</a></blockquote>
<script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>
    </div><!-- end of col-sm-4 -->
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      <p>Code worksheet #4. Made with <i class="fa fa-heart-o"></i> and <i class="fa fa-coffee"></i> Kauress 2019</p>
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