Angular JS マウスイベントの処理について

下記はマウス操作で座標を出力する処理です。
getCrossBrowserElementCoordsで処理している内容がよくわからず、教えていただけるでしょうか。
①下記はmouseEventが空だったら行われる処理だと思うのですが、そのような状況は起きるのか

      if (!mouseEvent)
      {
        mouseEvent = window.event;
      }

②if (mouseEvent.pageX || mouseEvent.pageY)以降の処理が具体的に何をしているのか理解できず、ご教授をお願いいたします。

HTML5
1<!DOCTYPE html>
2<html>
3<head>
4  <script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.20/angular.min.js"></script>
5  <script src="script.js"></script>
6</head>
7
8<body ng-app="mainModule">
9  <div ng-controller="mainController">
10    <h3>4. Mouse down, up, enter, leave, move, over</h3>
11    Move the mouse on the square<br />
12    <img src="images/square.png"
13         ng-mousedown="onMouseDown($event)"
14         ng-mouseup="onMouseUp($event)"
15         ng-mouseenter="onMouseEnter($event)"
16         ng-mouseleave="onMouseLeave($event)"
17         ng-mousemove="onMouseMove($event)"
18         ng-mouseover="onMouseOver($event)" /><br />
19    <strong>MOUSE DOWN RESULT:</strong> {{onMouseDownResult}}<br />
20    <strong>MOUSE UP RESULT:</strong> {{onMouseUpResult}}<br />
21    <strong>MOUSE ENTER RESULT:</strong> {{onMouseEnterResult}}<br />
22    <strong>MOUSE LEAVE RESULT:</strong> {{onMouseLeaveResult}}<br />
23    <strong>MOUSE MOVE RESULT:</strong> {{onMouseMoveResult}}<br />
24    <strong>MOUSE OVER RESULT:</strong> {{onMouseOverResult}}
25  </div>
26</body>
27</html>

AngularJS
1angular.module("mainModule", [])
2  .controller("mainController", function ($scope)
3  {
4    // Initialization
5    $scope.onFirstBtnClickResult = "";
6    $scope.secondBtnInput = "";
7    $scope.onDblClickResult = "";
8    $scope.onMouseDownResult = "";
9    $scope.onMouseUpResult = "";
10    $scope.onMouseEnterResult = "";
11    $scope.onMouseLeaveResult = "";
12    $scope.onMouseMoveResult = "";
13    $scope.onMouseOverResult = "";
14
15    // Utility functions
16
17    // Accepts a MouseEvent as input and returns the x and y
18    // coordinates relative to the target element.
19    var getCrossBrowserElementCoords = function (mouseEvent)
20    {
21      var result = {
22        x: 0,
23        y: 0
24      };
25
26      if (!mouseEvent)
27      {
28        mouseEvent = window.event;
29      }
30
31      if (mouseEvent.pageX || mouseEvent.pageY)
32      {
33        result.x = mouseEvent.pageX;
34        result.y = mouseEvent.pageY;
35      }
36      else if (mouseEvent.clientX || mouseEvent.clientY)
37      {
38        result.x = mouseEvent.clientX + document.body.scrollLeft +
39          document.documentElement.scrollLeft;
40        result.y = mouseEvent.clientY + document.body.scrollTop +
41          document.documentElement.scrollTop;
42      }
43
44      if (mouseEvent.target)
45      {
46        var offEl = mouseEvent.target;
47        var offX = 0;
48        var offY = 0;
49
50        if (typeof(offEl.offsetParent) != "undefined")
51        {
52          while (offEl)
53          {
54            offX += offEl.offsetLeft;
55            offY += offEl.offsetTop;
56
57            offEl = offEl.offsetParent;
58          }
59        }
60        else
61        {
62          offX = offEl.x;
63          offY = offEl.y;
64        }
65
66        result.x -= offX;
67        result.y -= offY;
68      }
69
70      return result;
71    };
72
73    var getMouseEventResult = function (mouseEvent, mouseEventDesc)
74    {
75      var coords = getCrossBrowserElementCoords(mouseEvent);
76      return mouseEventDesc + " at (" + coords.x + ", " + coords.y + ")";
77    };
78
79    // Event handlers
80    $scope.onFirstBtnClick = function () {
81      $scope.onFirstBtnClickResult = "CLICKED";
82    };
83
84    $scope.onSecondBtnClick = function (value) {
85      $scope.onSecondBtnClickResult = "you typed '" + value + "'";
86    };
87
88    $scope.onDblClick = function () {
89      $scope.onDblClickResult = "DOUBLE-CLICKED";
90    };
91
92    $scope.onMouseDown = function ($event) {
93      $scope.onMouseDownResult = getMouseEventResult($event, "Mouse down");
94    };
95
96    $scope.onMouseUp = function ($event) {
97      $scope.onMouseUpResult = getMouseEventResult($event, "Mouse up");
98    };
99
100    $scope.onMouseEnter = function ($event) {
101      $scope.onMouseEnterResult = getMouseEventResult($event, "Mouse enter");
102    };
103
104    $scope.onMouseLeave = function ($event) {
105      $scope.onMouseLeaveResult = getMouseEventResult($event, "Mouse leave");
106    };
107
108    $scope.onMouseMove = function ($event) {
109      $scope.onMouseMoveResult = getMouseEventResult($event, "Mouse move");
110    };
111
112    $scope.onMouseOver = function ($event) {
113      $scope.onMouseOverResult = getMouseEventResult($event, "Mouse over");
114    };
115  });