Using the alert function to show a message to the user is disruptive, since an alert dialog prevents the user from accessing the rest of the program's interface until the dialog is closed. Also, alert dialogs are displayed by the browser and hence cannot be styled to fit with the rest of the web application, which negatively affects user experience.

Some developers use alert during debugging, so left over calls to alert may also indicate unfinished code.

Use one of the HTML-based dialog box widgets offered by frameworks such as jQuery UI, and consider using a non-modal dialog box where possible.

The following code snippet uses alert to print a warning message if an element of a form is not correctly filled in. A preferable option would be to visually highlight the corresponding field to provide more context for the warning.

Mozilla Developer Network: Window.alert.