DOM element coordinate helpers

coords.js

/**
 * MIT License
 * @author Alexander Moon mainarx@gmail.com
 */

/**
 * An offset type definition
 * @typedef {Object} Offset
 * @property {number} top
 * @property {number} left
 */

/**
 * A point type defition
 * @typedef {Object} Point
 * @property {number} x
 * @property {number} y
 */

/**
 * An offset getter. Optionally gets an offset relative to given parent node
 * @param {HTMLElement} el - Target element, that offset is going to be
 * calculated for
 * @param {HTMLElement} [parent] - Relative element
 * @return Offset
 */
function offset(el, parent = null) {
  var top  = 0,
      left = 0;
  if (el.offsetParent) {
    do {
      top += el.offsetTop;
      left += el.offsetLeft;
    }
    while ((el = el.offsetParent));
    if (parent && document.body.contains(parent)) {
      var local = globalToLocal(parent, left, top);
      return {
        top: local.y,
        left: local.x
      }
    }
    return {top: top, left: left};
  }
}

/**
 * Function that converts a point in element's coordinate space to document's
 * @param {HTMLElement} el An element to base coordinates off
 * @param {number} lx Local X coordinate, within el-s coordinate space
 * @param {number} ly Local Y coordinate, within el-s coordinate space
 * @return Point
 */
function localToGlobal(el, lx, ly) {
  var op = offset(el);
  return ({
    x: Math.floor(lx + op.left),
    y: Math.floor(ly + op.top)
  });
}

/**
 * Function that converts a point in documents's coordinate space to coordinate
 * space of given element
 * @param {HTMLElement} el An element to base coordinates off
 * @param {number} gx Global X coordinate, within document's coordinate space
 * @param {number} gy Global Y coordinate, within document's coordinate space
 * @return Point
 */
function globalToLocal(el, gx, gy) {
  var op = offset(el);
  return {
    x: Math.floor(gx - op.left),
    y: Math.floor(gy - op.top)
  }
}

/**
 * Converts a point in coordinate space of source element, to a point in
 * coordinate space of destination element. Useful when you need to relocate
 * an element within DOM-tree, but keep it visually intact
 * @param {HTMLElement} src HTML element to base off coordinates
 * @param {HTMLElement} dest HTML element to do coordinate conversion for
 * @param {number} lx Local x coordinate
 * @param {number} ly Local y coordinate
 * @return Point
 */
function localToLocal(src, dest, lx, ly) {
  var srcOffset  = offset(src),
      destOffset = offset(dest);
  return {
    x: Math.floor(lx + srcOffset.left - destOffset.left),
    y: Math.floor(ly + srcOffset.top - destOffset.top)
  }
}