Can You Access a Function\'s Closure Programmatically in JavaScript?

Accessing the Closure of a Function

In JavaScript, functions form closures by retaining a hidden link to their enclosing scope. This raises the question: is it possible to access this closure programmatically when the function is available as a variable value?

The primary motivation behind this inquiry is theoretical, but a practical application could involve listing the properties of the closure. Consider the following code:

<code class="js">var x = (function(){
   var y = 5;
   return function() {
       alert(y);
   };
})();

//access y here with x somehow</code>

The goal is to access the variable y using the function x.

Accessing the Closure

In frontend environments, where you can execute your JavaScript in a prior script tag, one approach is to attach a MutationObserver. This observer waits for the script tag you want to examine (x in this case) to be inserted into the document and modifies its code to expose the desired functionality.

For example:

<code class="js">new MutationObserver((mutations, observer) => {
  // Find the target script tag
  const tamperTarget = document.querySelector('script + script');
  if (!tamperTarget) {
    return;
  }
  observer.disconnect();
  console.log('Target script getting tampered with');

  // Modify the script's content to expose the closure
  tamperTarget.textContent = tamperTarget.textContent.replace(
    'return function',
    'window.y = y; return function'
  );

  setTimeout(() => {
    console.log("Hacked into tamper target's script and found a y of", y);
    console.log('Could also have replaced the local y with another value');
  });
})
  .observe(document.body, { childList: true });</code>

This script attaches the observer to the document.body. When the script tag containing the x function is inserted, the observer triggers and replaces the line return function with window.y = y; return function. This exposes the y variable as a global variable (window.y).

Subsequently, you can access and manipulate the y variable as desired. This technique can be useful for debugging, testing, and experimentation with closures.

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