AssertionError [ERR_ASSERTION]: Invalid callback object specified.

This error occurs when a function that expects a callback is called with an invalid callback object. A callback is a function that is passed as an argument to another function and is executed when the main function completes its operation.

To resolve this error, you should check that the callback function is defined and has the correct syntax. You should also verify that the function that is calling the callback is passing the correct arguments.

Example:

function fetchData(callback) {
  if (typeof callback === 'function') {
    callback(null, data);
  } else {
    throw new AssertionError('Invalid callback object specified.');
  }
}

// Correct syntax for callback function
function myCallbackFunction(error, data) {
  if (error) {
    console.log(error);
  } else {
    console.log(data);
  }
}

// Call the fetchData function with a valid callback object
fetchData(myCallbackFunction);

// Call the fetchData function with an invalid callback object
fetchData('not a function');

In the above example, the `fetchData` function expects a callback function to be passed as an argument. If the callback is not a function, an `AssertionError` is thrown with the message "Invalid callback object specified."