Understanding Asynchronous Code in Layman’s terms

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I’m not sure where this function is being used, but the point of callbacks is that you pass them into some function that runs asynchronously; it stores your callback away, and when that function is done with whatever it needs to do, it will call your callback with the necessary parameters. An example from front-to-back is probably best.

Imagine we have a framework, and in it there is an operation that runs for a long time, fetching some data from the database.

function getStuffFromDatabase() {
  // this takes a long time
};

Since we don’t want it to run synchronously, we’ll allow the user to pass in a callback.

function getStuffFromDatabase(callback) {
  // this takes a long time
};

We’ll simulate taking a long time with a call to setTimeout; we’ll also pretend we got some data from the database, but we’ll just hardcode a string value.

function getStuffFromDatabase(callback) {
  setTimeout(function() {
    var results = "database data";
  }, 5000);
};

Finally, once we have the data, we’ll call the callback given to us by the user of the framework’s function.

function getStuffFromDatabase(callback) {
  setTimeout(function() {
    var results = "database data";
    callback(results);
  }, 5000);
};

As a user of the framework, you’d do something like this to use the function:

getStuffFromDatabase(function(data) {
  console.log("The database data is " + data);
});

So, as you can see data (same as response and postData in your example) came from the function that you pass your callback into; it gives that data to you when it knows what that data should be.

The reason you can’t set a value in your callback and use it outside the callback is because the callback itself doesn’t happen until later in time.

//  executed immediately  executed sometime in the future
//      |                  |       by getStuffFromDatabase
//      v                  v
getStuffFromDatabase(function(data) {
  var results = data; // <- this isn't available until sometime in the future!
});

console.log(results); // <- executed immediately

When the console.log runs, the assignment of var results hasn’t happened yet!

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