AJAX GET Request in JavaScript: Loading an External JavaScript File

This code creates a new XMLHttpRequest object and sets its properties to perform a GET request to load the 'smsc.js' file located in the 'js' directory. When the request is sent, the function sets an event listener to listen for changes in the request state. Once the state changes to 4 (which indicates that the response has been received), the function checks if the status code is between 200 and 299 (which indicates a successful response). If the response is successful, the function displays the response text in an alert box.