The Element.getBoundingClientRect() method returns a DOMRect object providing information about the size of an element and its position relative to the viewport.

None.

The returned value is a DOMRect object which is the smallest rectangle which contains the entire element, including its padding and border-width. The left, top, right, bottom, x, y, width, and height properties describe the position and size of the overall rectangle in pixels. Properties other than width and height are relative to the top-left of the viewport.

The width and height properties of the DOMRect object returned by the method include the padding and border-width, not only the content width/height. In the standard box model, this would be equal to the width or height property of the element + padding + border-width. But if box-sizing: border-box is set for the element this would be directly equal to its width or height.

The returned value can be thought of as the union of the rectangles returned by getClientRects() for the element, i.e., the CSS border-boxes associated with the element.

Empty border-boxes are completely ignored. If all the element's border-boxes are empty, then a rectangle is returned with a width and height of zero and where the top and left are the top-left of the border-box for the first CSS box (in content order) for the element.

The amount of scrolling that has been done of the viewport area (or any other scrollable element) is taken into account when computing the bounding rectangle. This means that the rectangle's boundary edges (top, right, bottom, left) change their values every time the scrolling position changes (because their values are relative to the viewport and not absolute).

If you need the bounding rectangle relative to the top-left corner of the document, just add the current scrolling position to the top and left properties (these can be obtained using window.scrollY and window.scrollX) to get a bounding rectangle which is independent from the current scrolling position.

This simple example retrieves the DOMRect object representing the bounding client rect of a simple <div> element, and prints out its properties below it.

<div></div>

div {
  width: 400px;
  height: 200px;
  padding: 20px;
  margin: 50px auto;
  background: purple;
}

let elem = document.querySelector("div");
let rect = elem.getBoundingClientRect();
for (const key in rect) {
  if (typeof rect[key] !== "function") {
    let para = document.createElement("p");
    para.textContent = `${key} : ${rect[key]}`;
    document.body.appendChild(para);
  }
}

Notice how the width/height are equal to its width/height + padding.

Also note how the values of x/left, y/top, right, and bottom are equal to the absolute distance from the relevant edge of the viewport to that side of the element, in each case.

This example demonstrates how bounding client rect is changing when document is scrolled.

<div id="example"></div>
<div id="controls"></div>

div#example {
  width: 400px;
  height: 200px;
  padding: 20px;
  margin: 50px auto;
  background: purple;
}

body {
  padding-bottom: 1000px;
}
p {
  margin: 0;
}

function update() {
  const container = document.getElementById("controls");
  const elem = document.getElementById("example");
  const rect = elem.getBoundingClientRect();

  container.innerHTML = "";
  for (const key in rect) {
    if (typeof rect[key] !== "function") {
      let para = document.createElement("p");
      para.textContent = `${key} : ${rect[key]}`;
      container.appendChild(para);
    }
  }
}

document.addEventListener("scroll", update);
update();

• getClientRects()
• MSDN: getBoundingClientRect
• MSDN: ClientRect, an earlier version of DOMRect