Contact Form 7 AJAX Callback

document.addEventListener( 'wpcf7mailsent', function( event ) {
  alert( "Fire!" );
}, false );

And the valid events are:

• wpcf7invalid — Fires when an Ajax form submission has completed successfully, but mail hasn’t been sent because there are fields with invalid input.
• wpcf7spam — Fires when an Ajax form submission has completed successfully, but mail hasn’t been sent because a possible spam activity has been detected.
• wpcf7mailsent — Fires when an Ajax form submission has completed successfully, and mail has been sent.
• wpcf7mailfailed — Fires when an Ajax form submission has completed successfully, but it has failed in sending mail.
• wpcf7submit — Fires when an Ajax form submission has completed successfully, regardless of other incidents.
  
  Source: https://contactform7.com/dom-events/