Compares the position of two elements in a DOM tree and returns -1, 0 or 1

dom-position.js

// Note: This code is obsolete. There's a cross-browser DOM method `compareDocumentPosition()` which does the same, but more detailed.
// See https://developer.mozilla.org/en-US/docs/Web/API/Node/compareDocumentPosition

function compareNodePosition( nodeA, nodeB, container ) {
  if ( !( container instanceof HTMLElement ) ) {
    container = document.body;
  }
  
  function getNodePositionArray( el, container ) {
    var arr = [];
    var parentNode = el;

    while ( parentNode !== container ) {
      arr.unshift( Array.from( parentNode.parentNode.childNodes ).indexOf( parentNode ) );
      parentNode = parentNode.parentNode;
      
      if ( parentNode === null ) {
        throw {
          message: "Could not find node in given container",
          node: el,
          container: container
        };
      }
    }

    return arr;
  }

  function comparePositionArray( a, b ) {
    if ( a.length === 0 && b.length === 0 ) {
      return 0;
    } else if ( a.length === 0 ) {
      return 1;
    } else if ( b.length === 0 ) {
      return -1;
    } else {
      if ( a[ 0 ] === b[ 0 ] ) {
        a.shift();
        b.shift();
        return comparePositionArray( a, b );
      } else if ( a[ 0 ] < b[ 0 ] ) {
        return -1;
      } else {
        return 1;
      }
    }
  }
  
  return comparePositionArray( getNodePositionArray( nodeA, container ), getNodePositionArray( nodeB, container ) );
}