Why can't JS private variables be accessed?

<code>function Customer(name) {  
    var risk = 0;  
    this.name = name;  
}  
var customer = new Customer("aa"); 
console.log(customer.name);  // aa
console.log(customer.risk); // undefined  </code>

Excuse me, why is customer.risk inaccessible but customer.name is accessible? Whose private variable is risk? Is it customer's? If so, why can't customer access his own private variable?