How do I code a polite alert?

A polite alert is a message which is presented by the screen reader after it has completed all queued announcements. Typically these should be used when you want to present a message to assistive technology without a user having to navigate to that message, but when the message is not important enough to warrant interrupting the screen reader's announcement of other content.

To create a polite alert, place an empty <div> on the web page and give that <div> an attribute of aria-live="polite". Then, when you wish to present the status message, inject the text into that <div>.

For a deeper dive into handling error alerts and other status messages, explore the "Forms: Labels, Cues, and Errors" course from Accessible Web Academy.