An alert box is a built-in feature in JavaScript that displays a small window with a message to the user. It's primarily used for providing information to the user, displaying warnings, or prompting the user for confirmation. The below approaches can be used to create a custom alert box in JavaScript:

Though jQuery UI has lots of APIs, you can use its dialog() API to create a custom alert box. Meanwhile, unlike the native JavaScript native alert() function, you can drag the alert box created with the dialog() API. We’ve imported jQuery, jQuery UI, and the CSS styles for jQuery UI into our code in the following code block.

Give the alert container an HTML class name. Create a div element for the alert box and append it to the alert container. Afterward, give it an HTML class name. Set the top position of the alert box using scrollTop. Set the left position of the alert box using scrollWidth and offsetWidth. Create an HTML h1 element for the alert title.

Give the alert container an HTML ID. Give the alert container an HTML class name. Create a div element for the alert box and append it to the alert container. Afterward, give it an HTML class name. Set the top position of the alert box using scrollTop. Set the left position of the alert box using scrollWidth and offsetWidth.