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Unless I end Ajax processing function by exit() or die(), the Ajax function receives desired and correct output but following by 0.
Any idea why is that happening?
Is that meant to be like that or it could be fixed?
add_action('wp_ajax_get_homepage', 'get_homepage');
add_action('wp_ajax_nopriv_get_homepage', 'get_homepage');
function get_homepage(){
echo "get_homepage ->";
exit();
}
I've read at AJAX in Plugins, but why is that necessary?
The default response from admin-ajax.php is,
die( '0' );
...by adding your own wp_die() or exit() or die() after returning your desired content prevents the default response from admin-ajax.php being returned as well.
It also generally means that your ajax call has succeeded.
Ultimately, to answer your question, it's meant to work this way. What you are doing by exiting after returning your content is the right thing to do.
die() or exit() after you echo your content/data. In the success/complete callback function of your ajax method, you can then manipulate the data and use it to populate the DOM. You can also do any additional error checking to validate data integrity. You can also do error checking in your PHP callback function prior to output. Essentially exiting after returning your response will ensure you don't also get the 0 along with it. By the way just because you receive a 0 response does not mean your ajax call was 100% successful. However its easy to tell when you inspect your output.
wp_die to other plugins or theme code that can hook onto said process, then, using die or exit is ok. If you intend to release your code as OSS or otherwise, where your code needs to play nice with others, then wp_die would be more preferable, agreed.