See
https://ajaxcontroltoolkit.codeplex.com/SourceControl/latest#AjaxControlToolkit/Scripts/MaskedEdit.js
It uses __.filter()__ and __.map()__ functions that are missing in IE8. Therefore they have to rewritten or documentation changed to support IE9+
Comments: The issue is fixed and the fix will be included in one of our future releases.
https://ajaxcontroltoolkit.codeplex.com/SourceControl/latest#AjaxControlToolkit/Scripts/MaskedEdit.js
It uses __.filter()__ and __.map()__ functions that are missing in IE8. Therefore they have to rewritten or documentation changed to support IE9+
Comments: The issue is fixed and the fix will be included in one of our future releases.