Oh! I found the solution while posting it but post the question to give the solution others. I just followed 'this' in 'mouseenter' event and found that 'this' is already the div 'top'. Then why should I tell 'this' to select the 'top'. Then I obsolete the 'top' and the final code is:
this.top = $('#rootDiv_' + root);
this.top.mouseenter(function () {
this.style.cursor = "pointer";
});