is this secure from browser extensions inspecting the secret value?

gistfile1.js

var Hasher = (function(){
  var secret = 42;
  
  var Hasher = function() {
  };

  Hasher.prototype.setSecret = function(s) {
    secret = s;
  };
  Hasher.prototype.hash = function(x) {
    return secret * x;
  };
  return Hasher;
  
})();

var h = new Hasher();
h.setSecret(321);
console.log(h.hash(7)); // prints 2247
console.log(h.secret);  // prints undefined

http://jsfiddle.net/nN2y7/