Accessing private member variables from prototype-defined functions

javascript private-members
morgancodes picture morgancodes · Jan 12, 2009 · Viewed 119.6k times · Source

Is there any way to make “private” variables (those defined in the constructor), available to prototype-defined methods?

TestClass = function(){
    var privateField = "hello";
    this.nonProtoHello = function(){alert(privateField)};
};
TestClass.prototype.prototypeHello = function(){alert(privateField)};

This works:

t.nonProtoHello()

But this doesn’t:

t.prototypeHello()

I’m used to defining my methods inside the constructor, but am moving away from that for a couple reasons.

Answer

Triptych picture Triptych · Jan 12, 2009

No, there's no way to do it. That would essentially be scoping in reverse.

Methods defined inside the constructor have access to private variables because all functions have access to the scope in which they were defined.

Methods defined on a prototype are not defined within the scope of the constructor, and will not have access to the constructor's local variables.

You can still have private variables, but if you want methods defined on the prototype to have access to them, you should define getters and setters on the this object, which the prototype methods (along with everything else) will have access to. For example:

function Person(name, secret) {
    // public
    this.name = name;

    // private
    var secret = secret;

    // public methods have access to private members
    this.setSecret = function(s) {
        secret = s;
    }

    this.getSecret = function() {
        return secret;
    }
}

// Must use getters/setters 
Person.prototype.spillSecret = function() { alert(this.getSecret()); };

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