// Mal ejemplo, aunque visiblemente más entendible
document.onmouseup = function (e) {
  document.body.innerHTML += `${e.pageX}, ${e.pageY} <br>`;
}

/*
document.addEventListener('mouseup', function(e){
  document.body.innerHTML += `${e.pageX}, ${e.pageY} <br>`;
});
*/

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External JavaScript

This Pen doesn't use any external JavaScript resources.