Find mouse position relative to element

For people using JQuery:

Sometimes, when you have nested elements, one of them with the event attached to it, it can be confusing to understand what your browser sees as the parent. Here, you can specify which parent.

You take the mouse position, and then subtract it from the parent element's offset position.

var x = evt.pageX - $('#element').offset().left;
var y = evt.pageY - $('#element').offset().top;

If you're trying to get the mouse position on a page inside a scrolling pane:

var x = (evt.pageX - $('#element').offset().left) + self.frame.scrollLeft();
var y = (evt.pageY - $('#element').offset().top) + self.frame.scrollTop();

Or the position relative to the page:

var x = (evt.pageX - $('#element').offset().left) + $(window).scrollLeft();
var y = (evt.pageY - $('#element').offset().top) + $(window).scrollTop();

Note the following performance optimisation:

var offset = $('#element').offset();
// Then refer to 
var x = evt.pageX - offset.left;

In this way, JQuery does not have to look up #element for each line.

Update

There is a newer, JavaScript-only version in an answer by @anytimecoder -- see also browser support for getBoundingClientRect().

The following calculates the mouse position relation to the canvas element:

const example = document.getElementById('example');

example.onmousemove = function(e) { 
    const x = e.pageX - e.currentTarget.offsetLeft; 
    const y = e.pageY - e.currentTarget.offsetTop; 
}

As I didn't find a jQuery-free answer that I could copy/paste, here's the solution I used:

document.getElementById('clickme').onclick = function clickEvent(e) {
      // e = Mouse click event.
      var rect = e.target.getBoundingClientRect();
      var x = e.clientX - rect.left; //x position within the element.
      var y = e.clientY - rect.top;  //y position within the element.
      console.log("Left? : " + x + " ; Top? : " + y + ".");
    }

#clickme {
  margin-top: 20px;
  margin-left: 100px;
  border: 1px solid black;
  cursor: pointer;
}

<div id="clickme">Click Me -<br>
(this box has margin-left: 100px; margin-top: 20px;)</div>

JSFiddle of full example

There is no answer in pure javascript that returns relative coordinates when the reference element is nested inside others which can be with absolute positioning. Here is a solution to this scenario:

function getRelativeCoordinates (event, referenceElement) {

  const position = {
    x: event.pageX,
    y: event.pageY
  };

  const offset = {
    left: referenceElement.offsetLeft,
    top: referenceElement.offsetTop
  };

  let reference = referenceElement.offsetParent;

  while(reference){
    offset.left += reference.offsetLeft;
    offset.top += reference.offsetTop;
    reference = reference.offsetParent;
  }

  return { 
    x: position.x - offset.left,
    y: position.y - offset.top,
  }; 

}