Can We Access the Closure of a Function in JavaScript?

Accessing the Closure of a Function

A closure is a function that forms a link to its enclosing scope, allowing it to access variables from that scope even after the function is executed. In JavaScript, this closure is hidden, making it challenging to access its properties programmatically.

Can we Access the Closure of a Function?

The answer is yes, but it requires some unconventional methods. One approach is to utilize a MutationObserver in a front-end environment. This technique involves modifying the source code of a script tag once it's inserted into the document, exposing the functionality you want to examine.

The example provided demonstrates this approach. It creates a MutationObserver that watches for the insertion of a new script tag. Once inserted, it replaces part of the script's code to expose an internal variable and its value.

<code class="js">new MutationObserver((mutations, observer) => {
  // Find the script tag to tamper with
  const tamperTarget = document.querySelector('script + script');
  if (!tamperTarget) {
    return;
  }
  observer.disconnect();
  tamperTarget.textContent = tamperTarget.textContent.replace(
    'return function',
    'window.y = y; return function'
  );
}).observe(document.body, { childList: true });</code>

Within the script tag that you want to investigate, the closure is accessed through the variable x. By modifying the script's code, you can expose the variable y from the closure to a global variable, allowing you to inspect its properties.

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

// Access y here with x somehow
console.log(window.y); // 5</code>

This technique provides a way to access and inspect the properties of a closure even though it's hidden within the function's scope.

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