Contact Form 7 AJAX Callback

In version 3.3 new jQuery custom event triggers were introduced:

New: Introduce 5 new jQuery custom event triggers

• wpcf7:invalid
• wpcf7:spam
• wpcf7:mailsent
• wpcf7:mailfailed
• wpcf7:submit

You can use wpcf7:invalid like the example below:

$(".wpcf7").on('wpcf7:invalid', function(event){  // Your code here});

Given the variety of responses on this topic the plugin developer seems to change their mind about how this should work every 5 minutes. Currently (Q1 2017) this is the working method:

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

And the valid events are:

• wpcf7invalid — Fires when an Ajax form submission has completedsuccessfully, but mail hasn’t been sent because there are fields withinvalid input.
• wpcf7spam — Fires when an Ajax form submission hascompleted successfully, but mail hasn’t been sent because a possiblespam activity has been detected.
• wpcf7mailsent — Fires when an Ajaxform submission has completed successfully, and mail has been sent.
• wpcf7mailfailed — Fires when an Ajax form submission has completedsuccessfully, but it has failed in sending mail.
• wpcf7submit — Fireswhen an Ajax form submission has completed successfully, regardlessof other incidents.

Sauce: https://contactform7.com/dom-events/

Sometimes it may not work, as Martin Klasson pointed out, only 'submit' event works, most probable because it's triggered by a form and bubbles up to the selected object. Also as I can understand, now events have other names, like "invalid.wpcf7", "mailsent.wpcf7", etc. In short, this should work:

jQuery('.wpcf7').on('invalid.wpcf7', function(e) {    // your code here});

More detailed explanation here: How to add additional settings on error in Contact form 7?