Core JavaScript 1.5 Reference:Global Objects:Error:constructor
From MDC
[edit] Summary
Returns a reference to the object's constructor.
[edit] Description
Returns a reference to the Error function that created the instance's prototype. The value of this property is a reference to the function itself, not a string containing the function's name. If this is desired, use the name property or the non-standard Function.prototype.name property.
Note that both an instance's constructor and name properties can be modified, although it is less likely that an Error's constructor will be redefined than it is for it to be renamed.