How to add private variable to this Javascript object literal snippet?

Question

Found this at MDC but how if I'd wanted to add a private variable to the

var dataset = {
    tables:{
        customers:{
            cols:[ /*here*/ ],
            rows:[ /*here*/ ]
        },
        orders:{
            cols:[ /*here*/ ],
            rows:[ /*here*/ ]
        }
    },
    relations:{
        0:{
            parent:'customers', 
            child:'orders', 
            keyparent:'custid', 
            keychild:'orderid',
            onetomany:true
        }
    }
}

The way I understand OOP in Javascript, I'd have access to dataset.tables.customers.cols[0] if such an item exists.
But if I wanted to place a private variable into customers, what would that look like?
Adding var index = 0; results in a runtime error.

Answer 1

You can't have "private" variables without a function involved. Functions are the only way to introduce a new scope in javascript.

But never fear, you can add functions in the right place to gain this sort of functionality with your object

var dataset = {
  tables: {
    customers:(function(){
      var privateVar = 'foo';
      return { 
        cols:[ /*here*/ ],
        rows:[ /*here*/ ]
      }
    }()),
    orders:{
      cols:[ /*here*/ ],
      rows:[ /*here*/ ]
    }
  },
  relations: [{
    parent:'customers', 
    child:'orders', 
    keyparent:'custid', 
    keychild:'orderid',
    onetomany:true
  }]
};

But this doesn't gain us much. This is still mostly just a bunch of literal objects. These types of "Private" variables have zero meaning since there are no methods - nothing that would actually read or otherwise use the variables in the scope we created by adding a function (a closure).

But if we had a method, that might actually start to become useful.

var dataset = {
  tables: {
    customers:(function(){
      var privateVar = 'foo';
      return { 
        cols:[ /*here*/ ],
        rows:[ /*here*/ ],
        getPrivateVar: function()
        {
          return privateVar;
        }
      };
    }()),
    orders:{
      cols:[ /*here*/ ],
      rows:[ /*here*/ ]
    }
  },
  relations: [{
    parent:'customers', 
    child:'orders', 
    keyparent:'custid', 
    keychild:'orderid',
    onetomany:true
  }]
};

alert( dataset.tables.customers.getPrivateVar() );