## Where Do Callback Function Parameters Come From in JavaScript?

Where Do Parameters in JavaScript Callback Functions Originate from?

In the context of callback functions, where do the parameters, such as the req and res in the Node.js example, come from? This question arises as callback functions are executed after being passed as parameters to another function.

Unraveling the Mystery of Callback Function Parameters

To understand the origin of these parameters, it's crucial to remember that they come from the same source as in regular non-callback functions: at the time of invocation.

Consider the following non-callback function:

<code class="javascript">function add(a, b) {
  return a + b;
}</code>

In this example, you know that a and b come from the invocation of the add function:

<code class="javascript">add(1, 2);</code>

The same principle applies to callback functions. Their parameters are passed to them when they are invoked.

The Invocation Context of Callback Functions

In the case of the Node.js example, the callback function is invoked when router.get is executed. Let's imagine a simplified definition of router.get:

<code class="javascript">router.get = function(endpoint, cb) {
  // Do something
  var request = {};
  var response = {};
  cb(request, response); // Invocation time
};</code>

When router.get is invoked with a specific endpoint and callback function, it creates request and response objects and passes them as parameters to the callback.

In your example, when .get is called, Node.js is responsible for passing request and response to your callback function, allowing you to utilize them without explicitly declaring them.

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