A technical lead who used to work for Laserfiche showed me this method which I find works the best in my opinion.

I have a alert message that needs to display if there are no courses for an instructor to choose from after lookups complete.


 

// Display the alert message when no courses are available to choose.
// The instructor id field will be blank if there are no courses
function ShowMessagesOnComplete() {
  var elm = $('.instructor-id input').val();
  if (!elm) {
  	$('.error-message').show();
  }
}

// Hide the alert message initially
function HideMessagesOnStart() {    
    $('.error-message').hide();
}

//Create promise wrappers for page load events
function WaitForDocumentReady() {
    const dfd = $.Deferred();
    $(document).ready(() => {
        console.log('[DOM Ready] Fired');
        dfd.resolve();
    });
    return dfd.promise();
}

// Creates a promise that waits for the lookup handlers to be in place. 
// It does this by listening for the onloadlookupfinished event and resolving
// the promise when it is fired.
function WaitForOnLoadLookupFinished() {
    const dfd = $.Deferred();
    const onLookupFinishedHandler = () => {
        console.log('[On Load Lookup Finished] Fired');
        // Show the alert if no courses are available after lookup finished
        ShowMessagesOnComplete();
        dfd.resolve();
    };
    const interuptHandler = () => {
        console.log('[Cancel On Load Lookup] Fired');
        dfd.resolve();
    };

    $(document).one('onloadlookupfinished', onLookupFinishedHandler);
    $(document).one('cancelonloadlookupwait', interuptHandler);

    return dfd.promise();
}

// Initialize promises by calling the $.when() method, which waits for all promises
// to be resolved before executing the callback function.
$.when(
    WaitForOnLoadLookupFinished(),
    WaitForDocumentReady().then(() => { 
    	HideMessagesOnStart();
}));