es3-set-environment-test.js

/**
 * Apply a function in needed environment.
 * Firefox only, educational purpose only.
 * by Dmitry Soshnikov <[email protected]>
 */

Function.prototype.setEnvironment = function(environment) {
  with (environment) {
    return eval(uneval(this));
  }
};

(function () {
  return x + y;
})
  .setEnvironment({x: 10, y: 20})
  .call(); // 30

@WebReflection, yeah, explicit toString works well in other browsers too. I just made it idiomatic uneval-eval sequence, and since the uneval isn't standardized (and behind the scene probably does toSource() rather than toString()), mentioned Firefox only.

Though, it's not for JS usage. This snippet is only for explaining generic concept, that without an environment function doesn't make sense. But once we provide the environment, the function has already runtime meaning.

Although, probably with this snippet it's possible to fake some private module data. If some function uses private vars of a module, and at the same time calls some this-methods, you may facilitate the private vars with your values.

var M = (function () {
  var privateData = 'privateUserId';
  return {
    send: function() {
      // ...
      // work with private user id data
      // ...
      console.log(privateData);
    }
  };
}());

M.send(); // private

M.send
  .setEnvironment({privateData: 'fakeUserId'})
  .call(M); // fake