Sorting array of custom objects in JavaScript

Say I have an array of Employee Objects:

var Employee = function(fname, age) {
     this.fname = fname;
     this.age = age;
 }
 
 var employees = [
     new Employee("Jack", "32"),
     new Employee("Dave", "31"),
     new Employee("Rick", "35"),
     new Employee("Anna", "33")
 ];
 

At this point, employees.sort() means nothing, because the interpreter does not know how to sort these custom objects. So I pass in my custom sort function.

employees.sort(function(employee1, employee2){
     return employee1.age > employee2.age;
 });
 

Now employees.sort() works dandy.

But what if I also wanted to control what field to be sorted on, passing it in somehow during runtime? Can I do something like this?

employees.sort(function(employee1, employee2, on){
     if(on === 'age') {
         return employee1.age > employee2.age;
     }
     return employee1.fname > employee2.fname;
 });
 

I can't get it to work, so suggestions? Design pattern based refactoring perhaps?

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