Create an AngularJS factory / object to hold an array

You could recreate this fairly simply without any need for the methods you are creating.

Javascript Objects are designed to do exactly what you are looking for here.

App.factory('userFilters', function() {
  return {};
});

Rather than using a getter and setter, you could instead get and set values with square bracket accessors.

// setting properties
userFilters['lastname'] = 'smith';
userFilters['firstname'] = 'bob';
userFilters['Mi'] = 'D';

// getting properties
userFilters['lastname']; // 'smith'
userFilters['firstname']; // 'smith'

If you want to be able to have full control of what happens at get/set time, you could look at intercepting these calls with internal getters and setters, providing you know the property names before hand.

Finally, you could also wrap your own get and set functions around the object in order to hide it. However, this would make more sense as a Service.

App.service('userFilters', function() {
  var store = {};

  this.get = function(key) {
    return store[key];
  };

  this.set = function(key, value) {
    store[key] = value;
  };
});

If it's important that your factory/service exposes an array then I would recommend sticking to using an object to store keys and values, but adding an array export method.

App.service('userFilters', function() {
  var store = {};

  this.toArray = function() {
    var records = [];
    return Object.keys(store).map(function(key) {
      records.push([key, store[key]]);
    });
  };

  this.get = function(key) {
    return store[key];
  };

  this.set = function(key, value) {
    store[key] = value;
  };
});

If you want to loop through the properties, you can use a for-in loop.

for(var key in userFilters) {
  var value = userFilters[key];
  console.log(key, value);
}

You can also check whether there any keys at all, using the Object.keys method.

Object.keys(userFilters);  // ['lastname', 'firstname', 'Mi']

This will return an array of all the keys in the object. If it has length 0, then you know it's empty.

In order to use an array for storage you will need to update both getData and setData methods and define data as an array []:

App.factory('userFilters', [function () {

    var data = [];

    var getData = function (field) {
        for(var i=0; i<data.length; i+=2) {
            if(data[i] == field) {
                return data[i+1];
            }
        }

        return null;
    };

    var setData = function (field, value) {
        data = data.concat([field, value]);
    };

    return {
        getData: getData,
        setData: setData
    }

}]);