Count down timer with circular progress bar

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Below is a sample snippet for the countdown timer with a circular progress bar that changes color as the value comes down.

Basically what we are doing is the follows: (refer inline comments in code for more details)

  • 4 additional div are absolutely positioned on top of the parent. Each represents a quadrant.
  • Initially the skew angle on all of them is 0 degree, so they are all fully visible and cover the entire parent. This hides the box-shadow of the parent and thus makes it look like a solid circle.
  • At every iteration, we modify the skew angle of each quadrant (div) such that the quadrants eventually become invisible one by one and thus revealing the box-shadow of parent.
  • The quadrants become invisible when the skew angle reaches +/- 90 degrees and so at each iteration the angle is calculated as (90deg / no. of steps covered in that quadrant).
  • As the progress moves past one quadrant to another, the box-shadow of the parent is changed to give the appearance of the progress bar changing its color.
  • The original CodePen uses the data-progress attribute’s value directly as the content of a pseudo-element. But that value gets incremented with every iteration. Since it is also used in calculation of the skew angles, I have left it as-is and used a separate field for the countdown timer. Content of pseudo-elements cannot be set using JS and so I have added another div for the timer text.
window.onload = function() {
  var progressbar = document.querySelector('div[data-progress]'),
    quad1 = document.querySelector('.quad1'),
    quad2 = document.querySelector('.quad2'),
    quad3 = document.querySelector('.quad3'),
    quad4 = document.querySelector('.quad4'),
    counter = document.querySelector('.counter');

  var progInc = setInterval(incrementProg, 1000); // call function every second

  function incrementProg() {
    progress = progressbar.getAttribute('data-progress'); //get current value
    progress++; // increment the progress bar value by 1 with every iteration
    progressbar.setAttribute('data-progress', progress); //set value to attribute
    counter.textContent = 100 - parseInt(progress, 10); // set countdown timer's value
    setPie(progress); // call the paint progress bar function based on progress value
    if (progress == 100) {
      clearInterval(progInc); // clear timer when countdown is complete
    }
  }

  function setPie(progress) {
    /* If progress is less than 25, modify skew angle the first quadrant */
    if (progress <= 25) {
      quad1.setAttribute('style', 'transform: skew(' + progress * (-90 / 25) + 'deg)');
    }

    /* Between 25-50, hide 1st quadrant + modify skew angle of 2nd quadrant */
    else if (progress > 25 && progress <= 50) {
      quad1.setAttribute('style', 'transform: skew(-90deg)'); // hides 1st completely
      quad2.setAttribute('style', 'transform: skewY(' + (progress - 25) * (90 / 25) + 'deg)');
      progressbar.setAttribute('style', 'box-shadow: inset 0px 0px 0px 20px orange'); 
    }

    /* Between 50-75, hide first 2 quadrants + modify skew angle of 3rd quadrant */
    else if (progress > 50 && progress <= 75) {
      quad1.setAttribute('style', 'transform: skew(-90deg)'); // hides 1st completely
      quad2.setAttribute('style', 'transform: skewY(90deg)'); // hides 2nd completely
      quad3.setAttribute('style', 'transform: skew(' + (progress - 50) * (-90 / 25) + 'deg)');
      progressbar.setAttribute('style', 'box-shadow: inset 0px 0px 0px 20px yellow');
    }

    /* Similar to above for value between 75-100 */
    else if (progress > 75 && progress <= 100) {
      quad1.setAttribute('style', 'transform: skew(-90deg)'); // hides 1st completely
      quad2.setAttribute('style', 'transform: skewY(90deg)'); // hides 2nd completely
      quad3.setAttribute('style', 'transform: skew(-90deg)'); // hides 3rd completely
      quad4.setAttribute('style', 'transform: skewY(' + (progress - 75) * (90 / 25) + 'deg)');
      progressbar.setAttribute('style', 'box-shadow: inset 0px 0px 0px 20px green');
    }
  }
}
div[data-progress] {
  box-sizing: border-box;
  position: relative;
  height: 200px;
  width: 200px;
  background: beige;
  border-radius: 50%;
  box-shadow: inset 0px 0px 0px 20px red;
  transition: all 1s;
  overflow: hidden;
}
.counter {
  position: absolute;
  height: 100%;
  width: 100%;
  top: 0%;
  left: 0%;
  text-align: center;
  line-height: 200px;
  border-radius: 50%;
  background: transparent;
  z-index: 4;
}
div > div {
  position: absolute;
  height: 50%;
  width: 50%;
  background: inherit;
  border-radius: 0%;
}
.quad1,
.quad2 {
  left: 50%;
  transform-origin: left bottom;
}
.quad3,
.quad4 {
  left: 0%;
  transform-origin: right top;
}
.quad1,
.quad4 {
  top: 0%;
}
.quad2,
.quad3 {
  top: 50%;
}
.quad1,
.quad3 {
  transform: skew(0deg); /* invisible at -90deg */
}
.quad2,
.quad4 {
  transform: skewY(0deg); /* invisible at 90deg */
}

/* Just for demo */

body {
  background: linear-gradient(90deg, crimson, indianred, purple);
}
div[data-progress] {
  margin: 40px auto;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/prefixfree/1.0.7/prefixfree.min.js"></script>
<div data-progress="0">
  <div class="quad1"></div>
  <div class="quad2"></div>
  <div class="quad3"></div>
  <div class="quad4"></div>
  <div class="counter">100</div>
</div>

In the below snippet, I have added a different background for each quadrant to provide a better visual illustration of what exactly is happening. 

window.onload = function() {
  var progressbar = document.querySelector('div[data-progress]'),
    quad1 = document.querySelector('.quad1'),
    quad2 = document.querySelector('.quad2'),
    quad3 = document.querySelector('.quad3'),
    quad4 = document.querySelector('.quad4'),
    counter = document.querySelector('.counter');

  var progInc = setInterval(incrementProg, 1000); // call function every second

  function incrementProg() {
    progress = progressbar.getAttribute('data-progress'); //get current value
    progress++; // increment the progress bar value by 1 with every iteration
    progressbar.setAttribute('data-progress', progress); //set value to attribute
    counter.textContent = 100 - parseInt(progress, 10); // set countdown timer's value
    setPie(progress); // call the paint progress bar function based on progress value
    if (progress == 100) {
      clearInterval(progInc); // clear timer when countdown is complete
    }
  }

  function setPie(progress) {
    /* If progress is less than 25, modify skew angle the first quadrant */
    if (progress <= 25) {
      quad1.setAttribute('style', 'transform: skew(' + progress * (-90 / 25) + 'deg)');
    }

    /* Between 25-50, hide 1st quadrant + modify skew angle of 2nd quadrant */
    else if (progress > 25 && progress <= 50) {
      quad1.setAttribute('style', 'transform: skew(-90deg)'); // hides 1st completely
      quad2.setAttribute('style', 'transform: skewY(' + (progress - 25) * (90 / 25) + 'deg)');
      progressbar.setAttribute('style', 'box-shadow: inset 0px 0px 0px 20px orange');
    }

    /* Between 50-75, hide first 2 quadrants + modify skew angle of 3rd quadrant */
    else if (progress > 50 && progress <= 75) {
      quad1.setAttribute('style', 'transform: skew(-90deg)'); // hides 1st completely
      quad2.setAttribute('style', 'transform: skewY(90deg)'); // hides 2nd completely
      quad3.setAttribute('style', 'transform: skew(' + (progress - 50) * (-90 / 25) + 'deg)');
      progressbar.setAttribute('style', 'box-shadow: inset 0px 0px 0px 20px yellow');
    }

    /* Similar to above for value between 75-100 */
    else if (progress > 75 && progress <= 100) {
      quad1.setAttribute('style', 'transform: skew(-90deg)'); // hides 1st completely
      quad2.setAttribute('style', 'transform: skewY(90deg)'); // hides 2nd completely
      quad3.setAttribute('style', 'transform: skew(-90deg)'); // hides 3rd completely
      quad4.setAttribute('style', 'transform: skewY(' + (progress - 75) * (90 / 25) + 'deg)');
      progressbar.setAttribute('style', 'box-shadow: inset 0px 0px 0px 20px green');
    }
  }
}
div[data-progress] {
  box-sizing: border-box;
  position: relative;
  height: 200px;
  width: 200px;
  background: beige;
  border-radius: 50%;
  box-shadow: inset 0px 0px 0px 20px red;
  transition: all 1s;
  overflow: hidden;
}
.counter {
  position: absolute;
  height: 100%;
  width: 100%;
  top: 0%;
  left: 0%;
  text-align: center;
  line-height: 200px;
  border-radius: 50%;
  background: transparent;
  z-index: 4;
}
div > div {
  position: absolute;
  height: 50%;
  width: 50%;
  border-radius: 0%;
}
.quad1,
.quad2 {
  left: 50%;
  transform-origin: left bottom;
}
.quad3,
.quad4 {
  left: 0%;
  transform-origin: right top;
}
.quad1, .quad4{
  top: 0%;
}
.quad2, .quad3{
  top: 50%;
}
.quad1, .quad3{
  transform: skew(0deg);
}
.quad2, .quad4{
  transform: skewY(0deg);
}

/* Just for demo */

body {
  background: linear-gradient(90deg, crimson, indianred, purple);
}
div[data-progress] {
  margin: 40px auto;
}
.quad1 {
  background: blue;
}
.quad2 {
  background: pink;
}
.quad3 {
  background: tan;
}
.quad4 {
  background: teal;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/prefixfree/1.0.7/prefixfree.min.js"></script>
<div data-progress="0">
  <div class="quad1"></div>
  <div class="quad2"></div>
  <div class="quad3"></div>
  <div class="quad4"></div>
  <div class="counter">100</div>
</div>

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