WP Admin Notice is a simple object use to show user notifications.

wp-admin-notice.js

/**
 * WP Admin Notice with pure JavaScript
 * 
 * This adds a dismissible admin notice to the WP admin with pure JavaScript.
 * No jQuery or other dependencies. The admin notice will have a Dismiss icon
 * just like the regular WP admin notices. When the icon is clicked, the message
 * will go away.
 * 
 * This script is and Object-Oriented implementation of @Isabelc post (see link below).
 * 
 * @see {@link https://isabelcastillo.com/wp-admin-notice-javascript Show WP Admin Notice With Pure JavaScript}
 * @see {@link https://developer.wordpress.org/reference/hooks/admin_notices Admin Notices - WP Code Reference}
 */
let Notification = Object.create(null);

/**
 * Define the notice level constants:
 * 
 * - error: will display the message with a white background and a red left border.
 * - warning: will display the message with a white background and a yellow/orange left border.
 * - success: will display the message with a white background and a green left border.
 * - info: will display the message with a white background a blue left border.
 */
Object.defineProperties(Notification, {
    INFO: {
        value: 'info',
        writable: false,
        enumerable: true,
    },
    SUCCESS: {
        value: 'success',
        writable: false,
        enumerable: true,
    },
    ERROR: {
        value: 'error',
        writable: false,
        enumerable: true,
    },
    WARNING: {
        value: 'warning',
        writable: false,
        enumerable: true,
    },
});

/**
 * Shows info message.
 * 
 * @param msg The message to be shown.
 */
Notification.info = function info(msg) {
    this.msg(msg, Notification.INFO);
}

/**
 * Shows success message.
 *
 * @param msg The message to be shown.
 */
Notification.success = function success(msg) {
    this.msg(msg, Notification.SUCCESS);
}

/**
 * Shows error message.
 *
 * @param msg The message to be shown.
 */
Notification.error = function error(msg) {
    this.msg(msg, Notification.ERROR);
}

/**
 * Shows warning message.
 *
 * @param msg The message to be shown.
 */
Notification.warning = function warning(msg) {
    this.msg(msg, Notification.WARNING);
}

/**
 * Create and show a dismissible admin notice
 */
Notification.msg = function msg(msg, level) {
    /* create notice div */
    var div = document.createElement('div');
    div.classList.add('notice', 'notice-' + level);

    /* create paragraph element to hold message */
    var p = document.createElement('p');

    /* Add message text */
    p.appendChild(document.createTextNode(msg));

    /* Add the whole message to notice div */
    div.appendChild(p);

    /* Create Dismiss icon */
    var b = document.createElement('button');
    b.setAttribute('type', 'button');
    b.classList.add('notice-dismiss');

    /* Add screen reader text to Dismiss icon */
    var bSpan = document.createElement('span');
    bSpan.classList.add('screen-reader-text');
    bSpan.appendChild(document.createTextNode('Dismiss this notice'));
    b.appendChild(bSpan);

    /* Add Dismiss icon to notice */
    div.appendChild(b);

    /* Insert notice after the first h1 */
    var h1 = document.getElementsByTagName('h1')[0];
    h1.parentNode.insertBefore(div, h1.nextSibling);

    /* Make the notice dismissable when the Dismiss icon is clicked */
    b.addEventListener('click', function () {
        div.parentNode.removeChild(div);
    });
}