How do I create JavaScript array (JSON format) dynamically?

I am trying the create the following:

var employees = {
  "accounting": [ // accounting is an array in employees.
    {
      "firstName": "John", // First element
      "lastName": "Doe",
      "age": 23
    },

    {
      "firstName": "Mary", // Second Element
      "lastName": "Smith",
      "age": 32
    }
  ] // End "accounting" array.                                  

} // End Employees

I started with

 var employees = new Array();

How do I continue to create the array dynamically (might change firstName with variable)? I don't seem to get the nested array right.