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css-variable-demo
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demo-6.html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    * {
      box-sizing: border-box;
    }

    *:before, *:after {
      box-sizing: inherit;
    }

    html, body {
      width: 100%;
      height: 100vh;
      margin: 0;
      padding: 0;
      font-family: Arial, sans-serif;
      line-height: 1.6;
    }

    .container {
      --blend-mode: multiply;
      --bg-color: #ff0000;
      --page-overflow: hidden;
      width: 100%;
      height: 100vh;
      overflow: var(--page-overflow);
      position: relative;
      background-color: var(--bg-color);
      background-image: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/234228/blending-image.jpg);
      background-repeat: no-repeat;
      background-position: top center;
      background-size: cover;
      background-blend-mode: var(--blend-mode);
    }

    .toggle-controls {
      position: absolute;
      top: 0;
      left: 0;
      width: 150px;
      padding: 1em;
      background-color: rgba(0, 0, 0, 0.5);
      z-index: 2;
    }
    .toggle-controls .toggle-controls__button {
      --webkit-appearance: none;
      --moz-appearance: none;
      padding: 0.5em;
      background-color: transparent;
      color: white;
      text-transform: uppercase;
      text-align: center;
      cursor: pointer;
    }

    .controls {
      --left-pos: translate(-250px);
      --controls-display: hidden;
      --transparency: 0;
      position: absolute;
      top: 125px;
      left: 0;
      -webkit-transform: var(--left-pos);
              transform: var(--left-pos);
      opacity: var(--transparency);
      max-width: 200px;
      overflow: var(--page-overflow);
      padding: 1em;
      background-color: white;
      transition: opacity 1s, -webkit-transform 1s;
      transition: transform 1s, opacity 1s;
      transition: transform 1s, opacity 1s, -webkit-transform 1s;
    }
    .controls label {
      display: inline-block;
      width: 100%;
      margin-bottom: 0.5em;
      font-weight: bold;
      color: #444;
    }
    .controls label:last-of-type {
      text-transform: uppercase;
      border-bottom: 3px dotted #444;
      padding-bottom: 0.5em;
    }
    .controls .color-picker {
      border: none;
      width: 100px;
      height: 40px;
      margin-top: 0.5em;
    }
  </style>
</head>
<body>
  <div class="container">
    <div class="toggle-controls">
      <button class="toggle-controls__button" type="button">Click to Choose Blend &amp; Color</button>
    </div>
    <div class="controls">
      <label for="multiply">
        <input type="radio" id="multiply" name="blends" value="multiply"> Multiply</label>
      <label for="overlay">
        <input type="radio" id="overlay" name="blends" value="overlay"> Overlay</label>
      <label for="screen">
        <input type="radio" id="screen" name="blends" value="screen"> Screen</label>
      <label for="darken">
        <input type="radio" id="darken" name="blends" value="darken"> Darken</label>
      <label for="lighten">
        <input type="radio" id="lighten" name="blends" value="lighten"> Lighten</label>
      <label for="color-dodge">
        <input type="radio" id="color-dodge" name="blends" value="color-dodge"> Color-Dodge</label>
      <label for="color-burn">
        <input type="radio" id="color-burn" name="blends" value="color-burn"> Color-Burn</label>
      <label for="soft-light">
        <input type="radio" id="soft-light" name="blends" value="soft-light"> Soft-Light</label>
      <label for="hard-light">
        <input type="radio" id="hard-light" name="blends" value="hard-light"> Hard-Light</label>
      <label for="exclusion">
        <input type="radio" id="exclusion" name="blends" value="exclusion"> Exclusion</label>
      <label for="difference">
        <input type="radio" id="difference" name="blends" value="difference"> Difference</label>
      <label for="hue">
        <input type="radio" id="hue" name="blends" value="hue"> Hue</label>
      <label for="saturation">
        <input type="radio" id="saturation" name="blends" value="saturation"> Saturation</label>
      <label for="color">
        <input type="radio" id="color" name="blends" value="color"> Color</label>
      <label for="luminosity">
        <input type="radio" id="luminosity" name="blends" value="luminosity"> Luminosity</label>
      <label for="normal">
        <input type="radio" id="normal" name="blends" value="normal"> Reset</label>

      <input type="color" class="color-picker" id="colorPicker" value="#ff0000">
    </div>
  </div>

  <script>
    //function to open and close the sidebar with the controls (with the click to choos button)
    const toggleControls = (elem, container, isOpen) => {
      //if the sidebar is open these values applied to
      //the appropriate CSS variables will close it
      if(isOpen) {
        elem.style.setProperty('--left-pos', 'translate(-250px)');
        elem.style.setProperty('--controls-display', 'hidden');
        elem.style.setProperty('--transparency', '0');
        container.style.setProperty('--page-overflow', 'hidden');
      }
      //otherwise the sidebar is closed and this code opens it
      else {
        elem.style.setProperty('--controls-display', 'visible');
        elem.style.setProperty('--transparency', '1');
        elem.style.setProperty('--left-pos', 'translate(0)');
        container.style.setProperty('--page-overflow', 'auto');
      }
    };

    //function to select a blend mode
    const pickBlendMode = (blendMode, elem) => {
      let chosenMode;
      //if the radio button is checked, grab its value and set it
      //as value of the --blend-mode CSS variable
      if (blendMode.checked) {
        chosenMode = blendMode.value;
        elem.style.setProperty('--blend-mode', chosenMode);
      }
    };

    //function to select background color from color picker
    //and use it to set the value of the --bg-color CSS variable
    const setBgColor = (picker, elem) => {
      let chosenColor = picker.value;
      elem.style.setProperty('--bg-color', chosenColor);
    };

    //vars
    const colorPicker = document.querySelector("#colorPicker"),
      toggleButton = document.querySelector('.toggle-controls__button'),
      controlsSidebar = document.querySelector('.controls'),
      docElement = document.querySelector('.container'),
      //turn blends from node list of radio buttons into array, so you can use
      //array methods on this variable
      blends = [].slice.call(document.getElementsByName("blends"));

    //picking blend modes by looping over the radio buttons,
    //attaching click events and calling the appropriate function
    //defined above
    blends.forEach((radioBtn) => {
      radioBtn.addEventListener('click', () => pickBlendMode(radioBtn, docElement));
    });

    //picking a color: attach a listener for when users change color on the
    //color picker
    colorPicker.addEventListener('change', () => setBgColor(colorPicker, docElement));

    //toggling controls panel on button click
    toggleButton.addEventListener('click', () => {

      //retrieve the current value of the --left-pos variable
      const cssStyles = getComputedStyle(controlsSidebar);
      let cssVal = String(cssStyles.getPropertyValue('--left-pos')).trim();

      //hide or show the sidebar with the controls
      //depending on the value of cssVal: -250px means
      //the sidebar is closed
      //(the parameter isOpen in the toggleControls() function is false),
      //therefore the button click
      //needs to open it, otherwise the button click will
      //close it
      if(cssVal === 'translate(-250px)') {
        toggleControls(controlsSidebar, docElement, false);
      } else {
          toggleControls(controlsSidebar, docElement, true);
        }
    });
  </script>
</body>
</html>