Measure distance between two HTML elements' centers

If I have HTML elements as follows:

<div id="x"></div>

<div id="y" style="margin-left:100px;"></div>

...how do I find the distance between them in pixels using JavaScript?

Get their positions, and use the Pythagorean Theorem to determine the distance between them...

 function getPositionAtCenter(element) {
   const {top, left, width, height} = element.getBoundingClientRect();
   return {
     x: left + width / 2,
     y: top + height / 2
   };
 }

function getDistanceBetweenElements(a, b) {
  const aPosition = getPositionAtCenter(a);
  const bPosition = getPositionAtCenter(b);

  return Math.hypot(aPosition.x - bPosition.x, aPosition.y - bPosition.y);  
}

const distance = getDistanceBetweenElements(
  document.getElementById("x"),
  document.getElementById("y")
);

If you browser doesn't support Math.hypot(), you can use instead:

Math.sqrt(
  Math.pow(aPosition.x - bPosition.x, 2) + 
  Math.pow(aPosition.y - bPosition.y, 2) 
);

The Pythagorean Theorem relates to the relationship between the sides of a right-angled triangle.

The elements are plotted on a Cartesian coordinate system (with origin in top left), so you can imagine a right-angled triangle between the elements' coordinates (the unknown side is the hypotenuse).

You can modify the equation to get the value of c by getting the square root of the other side.

Then, you simply plug the values in (the x and y are the differences between the elements once their centers are determined) and you will find the length of the hypotenuse, which is the distance between the elements.