今天在看完xtree.js代碼后，發現一個問題。在執行javascript的document.write(obj)方法時，如果obj實現了prototype的toString()方法，document.wirte方法將能自動調用obj的toString()方法。代碼如下：
???/*----------?? 樹構造代碼 ---------*/
??????? if (document.getElementById) {
?????????? var tree = new WebFXTree('Root');
?????????? tree.setBehavior('classic');
?????????? var a = new WebFXTreeItem('1');
?????????? tree.add(a);
?????????? var b = new WebFXTreeItem('1.1');
?????????? a.add(b);
????????? b.add(new WebFXTreeItem('1.1.1'));
????????? b.add(new WebFXTreeItem('1.1.2'));
????????? b.add(new WebFXTreeItem('1.1.3'));
????????? var f = new WebFXTreeItem('1.1.4');
????????? b.add(f);

???????? document.write(tree);

/*------------------ xtree.js代碼片段 ----------*/

WebFXTree.prototype.toString = function() {
alert("it's here!");
?var str = "<div id=\"" + this.id + "\" ondblclick=\"webFXTreeHandler.toggle(this);\" class=\"webfx-tree-item\" onkeydown=\"return webFXTreeHandler.keydown(this, event)\">" +
??"<img id=\"" + this.id + "-icon\" class=\"webfx-tree-icon\" src=\"" + ((webFXTreeHandler.behavior == 'classic' && this.open)?this.openIcon:this.icon) + "\" onclick=\"webFXTreeHandler.select(this);\">" +
??"<a href=\"" + this.action + "\" id=\"" + this.id + "-anchor\" onfocus=\"webFXTreeHandler.focus(this);\" onblur=\"webFXTreeHandler.blur(this);\"" +
??(this.target ? " target=\"" + this.target + "\"" : "") +
??">" + this.text + "</a></div>" +
??"<div id=\"" + this.id + "-cont\" class=\"webfx-tree-container\" style=\"display: " + ((this.open)?'block':'none') + ";\">";
?var sb = [];
?for (var i = 0; i < this.childNodes.length; i++) {
??sb[i] = this.childNodes[i].toString(i, this.childNodes.length);
?}
?this.rendered = true;
?return str + sb.join("") + "</div>";
};

經過跟蹤，證實在執行document.write(tree)時，調用了WebFXTree.prototype.toString 方法。