Question

I have following setup. method and method1 both are public methods of class Student. But why ca only “method” access private variable p

​function Student() {
    var p = 10;
    this.method = function() {
        document.write(p);
    }; 
};

Student.prototype.method1 = function() {
    document.write('here');
    document.write(p);
};

var s = new Student();
s.method();
s.method1();
​

How does it make sense, I mean is it “a public method that does not have access to private members!”

Answer 1

There is no magic going on with JavaScript’s prototypal inheritance.

Student is still a function and p is local to that function. It cannot be accessed from outside code in any way. method can access p because it is defined inside Student and therefore forms a closure, but method1 is declared outside Student‘s scope.

Assigning a function to another function’s prototype cannot give it access to its local variables.

Consider this example:

var p = 41;

function foo() {
    console.log(p);
}

​function Student(){
    var p = 10;
};

Student.prototype.bar = foo;

var s = new Student();
s.bar();
foo();

You might think that because foo is invoked as an object method it could have access to the local variables, but that’s just not the case. The only value that is determined dynamically is this, it is a special keyword. All other variables are strictly defined through the scope chain.