Get bounding client rectangle without borders

I made a function that transforms mouse coordinates to canvas pixel coordinates:

/* Returns pixel coordinates according to the pixel that's under the mouse cursor**/
HTMLCanvasElement.prototype.relativeCoords = function(event) {
  var x,y;
  //This is the current screen rectangle of canvas
  var rect = this.getBoundingClientRect();

  //Recalculate mouse offsets to relative offsets
  x = event.clientX - rect.left;
  y = event.clientY - rect.top;
  //Also recalculate offsets of canvas is stretched
  var width = rect.right - rect.left;
  //I use this to reduce number of calculations for images that have normal size 
  if(this.width!=width) {
    var height = rect.bottom - rect.top;
    //changes coordinates by ratio
    x = x*(this.width/width);
    y = y*(this.height/height);
  } 
  //Return as an array
  return [x,y];
}

You can see demonstration of the pixel coordinate calculation. The problem is that the solutions fails for images having border property set.

How can I subtract the border width from rectangle? Performance does matter, as this calculation is often performed during mouse move events.

getComputedStyle contains the information you desire:

Fetch the border information once at the beginning of your app after the canvas border has been set.

// get a reference to the canvas element
var canvas=document.getElementById('yourCanvasId');

// get its computed style
var styling=getComputedStyle(canvas,null);

// fetch the 4 border width values
var topBorder=styling.getPropertyValue('border-top-width');
var rightBorder=styling.getPropertyValue('border-right-width');
var bottomBorder=styling.getPropertyValue('border-bottom-width');
var leftBorder=styling.getPropertyValue('border-left-width');

If you scope these border-width variables app-wide, you can use these prefetched variables in your HTMLCanvasElement.prototype.relativeCoords.

Good luck with your project!