en.javascript.info/2-ui/1-document/11-coordinates/1-find-point-coordinates/solution.md

Outer corners

Outer corners are basically what we get from elem.getBoundingClientRect().

Coordinates of the upper-left corner answer1 and the bottom-right corner answer2:

let coords = elem.getBoundingClientRect();

let answer1 = [coords.left, coords.top];
let answer2 = [coords.right, coords.bottom];

Left-upper inner corner

That differs from the outer corner by the border width. A reliable way to get the distance is clientLeft/clientTop:

let answer3 = [coords.left + field.clientLeft, coords.top + field.clientTop];

Right-bottom inner corner

In our case we need to substract the border size from the outer coordinates.

We could use CSS way:

let answer4 = [
  coords.right - parseInt(getComputedStyle(field).borderRightWidth),
  coords.bottom - parseInt(getComputedStyle(field).borderBottomWidth)
];

An alternative way would be to add clientWidth/clientHeight to coordinates of the left-upper corner. That's probably even better:

let answer4 = [
  coords.left + elem.clientLeft + elem.clientWidth,
  coords.top + elem.clientTop + elem.clientHeight
];