Accessing variables trapped by closure

javascript

I was wondering if there is any way to access variables trapped by closure in a function from outside the function; e.g. if I have:


A = function(b) {
    var c = function() {//some code using b};
    foo: function() {
        //do things with c;
    }
}

is there any way to get access to c in an instance of A. Something like:


var a_inst = new A(123);
var my_c = somejavascriptmagic(a_inst);

A simple eval inside the closure scope can still access all the variables:

function Auth(username)
{
  var password = "trustno1";
  this.getUsername = function() { return username }
  this.eval = function(name) { return eval(name) }
}

auth = new Auth("Mulder")
auth.eval("username") // will print "Mulder"
auth.eval("password") // will print "trustno1"

But you cannot directly overwrite a method, which is accessing closure scope (like getUsername()), you need a simple eval-trick also:

auth.eval("this.getUsername = " + function() {
  return "Hacked " + username;
}.toSource());
auth.getUsername(); // will print "Hacked Mulder"

Variables within a closure aren't directly accessible from the outside by any means. However, closures within that closure that have the variable in scope can access them, and if you make those closures accessible from the outside, it's almost as good.

Here's an example:

var A = function(b) {
    var c = b + 100;
    this.access_c = function(value) {
        // Function sets c if value is provided, but only returns c if no value
        // is provided
        if(arguments.length > 0)
            c = value;
        return c;
    };
    this.twain = function() {
        return 2 * c;
    };
};
var a_inst = new A(123);
var my_c = a_inst.access_c();
// my_c now contains 223
var my_2c = a_inst.twain();
// my_2c contains 446
a_inst.access_c(5);
// c in closure is now equal to 5
var newer_2c = a_inst.twain();
// newer_2c contains 10

Hopefully that's slightly useful to you...

Answers above are correct, but they also imply that you'll have to modify the function to see those closed variables.

Redefining the function with the getter methods will do the task. You can do it dynamically. See the example below

function alertMe() {
    var message = "Hello world"; 
    console.log(message);
}

//adding the getter for 'message'
var newFun = newFun.substring(0, newFun.lastIndexOf("}")) + ";" + "this.getMessage = function () {return message;};" + "}";

//redefining alertMe
eval(newFun);

var b = new alertMe();

now you can access message by calling b.getMesage()

Of course you'll have to deal with multiple calls to alertMe, but its just a simple piece of code proving that you can do it.

Accessing variables trapped by closure

Related

Recent Posts