JavaScript module pattern: Function defines private variables and functions. Privileged functions can access private variables and functions via closure. The function returns the privileged functions or stores them in accessible place.

module_pattern.js

var serial_maker = function(){
  
  // produce an object that produces unique strings.
  
    var prefix = '';
    var seq = 0;
    return {
        set_prefix: function (p) {
            prefix = String(p);
        },
        set_seq: function(s) {
            seq = s;
        },
        gen_serial: function () {
            result = prefix + seq;
            seq += 1;
            return result;
        }
    };
};

var x = serial_maker();
x.set_prefix('abc');
x.set_seq(12345);
var serial_1 = x.gen_serial();
console.log(serial_1)                 // => 'abc12345'

// pass 'gen_serial' to 3rd party
var create_serial = x.gen_serial;

// 'create_serial' can still generate serial
var serial_2 = create_serial();
console.log(serial_2)                 // => 'abc12346'

// 'create_serial' can not set prefix or sequence
create_serial.set_prefix('fff');      // TypeError: object has no method 'set_prefix'
create_serial.set_seq(987654);        // TypeError: object has no method 'set_seq'