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                <!-- You can ignore this part -->
<input type="number" placeholder="angle (deg)" value="90"/>
  <circle cx="100" cy="100" r="0" />


                /* You can ignore this part, too */
svg, input {
  display: block;
  margin: 2px;

svg {
  width: 200px;
  height: 200px;
  stroke: #000;
  stroke-width: 2px;
  fill: transparent;


                // Simpler alternative to using SVG arcs:
// More about dash arrays:
// Setup DOM elements.
const input = document.querySelector('input');
const circle = document.querySelector('circle');

// All calculations are within 'run' function.
const run = () => {
  // 1. Get angle from input field.
  let angle = parseFloat(input.value) || 0;

  // 2. Radius of SVG circle.
  const radius = 50;
  const circumference = 2 * Math.PI * radius;

  // 3. First, 1/4 of circumfence of 90 degrees. To start from top of the view,
  //    we must rotate it by 90 degrees. By default circle will start on the right.
  //    Stroke offset effectively rotates the circle.
  // 4. Second, calculate dash array. We need dash array containing only two parts -
  //    visible dash, and invisible dash.
  //    Visible dash should have length of the chosen angle. Full circle is 360 degrees,
  //    and this 360 degrees does also equal the entire circumference. We want just a part of
  //    this entire circle to be visible - (angle / 360 degrees) returns a percentage value
  //    (between 0.0 and 1.0) of how much circumference should be visible.
  //    Hence, we then multiply (angle / 360) times the entire circumference.
  const strokeOffset = (1 / 4) * circumference;
  const strokeDasharray = (angle / 360) * circumference;

  // 5. Set circle radius
  circle.setAttribute('r', 50);
  // 6. Create dash array of two elements (combined they must equal the entire circumference).
  //    First has the length of visible portion. Second, the remaining part.
  circle.setAttribute('stroke-dasharray', [
    circumference - strokeDasharray
  // 7. (Optional) Rotate circle to start from the top.
  circle.setAttribute('stroke-dashoffset', strokeOffset);

// Run and update DOM
input.addEventListener('keyup', run);