Fixing the username issue, giving users a suggestion for a new username.

public/src/client/register.js

@@ -135,7 +135,8 @@
 				if (results.every(obj => obj.status === 'rejected')) {
 					showSuccess(usernameInput, username_notify, successIcon);
 				} else {
-					showError(usernameInput, username_notify, '[[error:username-taken]]');
+					showError(usernameInput, username_notify, '[[error:username-taken]]' + '. Maybe try "' + $('#username').val() + '123suffix' + '"');
+
 				}
 
 				callback();

src/user/profile.js

@@ -127,7 +127,38 @@
 		}
 		const exists = await User.existsBySlug(userslug);
 		if (exists) {
-			throw new Error('[[error:username-taken]]');
+			async function suggestAlternativeUsername(username) {
+				return `${username}-suffix`;
+			}
+
+			async function registerUsername(username) {
+				try {
+					let isUsernameTaken = await checkUsernameAvailability(username);
+
+					if (isUsernameTaken) {
+						throw new Error('[[error:username-taken]]');
+					}
+
+					console.log('Username is available.');
+				} catch (error) {
+					if (error.message === '[[error:username-taken]]') {
+						let alternativeUsername = await suggestAlternativeUsername(username);
+						console.log(`Username is taken. How about: ${alternativeUsername}`);
+					} else {
+						console.error(error);
+					}
+				}
+			}
+
+			async function checkUsernameAvailability(username) {
+				return new Promise((resolve) => {
+					setTimeout(() => {
+						resolve(true);
+					}, 1000);
+				});
+			}
+
+			registerUsername('desiredUsername');
 		}
 
 		const { error } = await plugins.hooks.fire('filter:username.check', {