## Where Do Input Variables in JavaScript Callback Functions Come From?

Javascript Callback Functions: Unraveling the Origin of Input Variables

When dealing with callback functions in JavaScript, a common question arises: where do the variables within the callback function originate? Let's delve into the answer, using a Node.js example for clarity.

In an example like:

<code class="javascript">router.get('/', function(req, res){
    res.render('index', {});
});</code>

The callback function takes two parameters, req and res. Understanding their origin is crucial.

Just like any other function invocation, the parameters of a callback function are provided when it's called. In this case, req and res are passed to the callback function by the router.get method.

Imagine the simplified version of router.get as:

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

When router.get is invoked with an endpoint and a callback, it creates request and response objects and passes them as parameters to the callback function. This effectively populates the req and res variables within the callback.

So, while the callback function is not immediately executed when passed as a parameter to another function, its parameters are established at the time of invocation. Just like in any other function, the parameters originate from the function call itself.

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