A WordPress detail that can silently break your AJAX

Most WordPress AJAX bugs aren't complex. They're one missing line.

The problem

function handle_ajax_raw() {
    echo 'Done';
}

add_action( 'wp_ajax_custom_action', 'handle_ajax_raw' );

This looks fine. But WordPress doesn't stop after your function runs. It may append extra output, debug info, or theme markup to the response. Your frontend receives a dirty string instead of clean data.

The fix

function handle_ajax_proper() {
    echo 'Done';
    wp_die();
}

add_action( 'wp_ajax_custom_action', 'handle_ajax_proper' );

wp_die() terminates execution immediately after your response. Nothing else gets appended.

Better: use wp_send_json_success()

function handle_ajax_json() {
    wp_send_json_success( array( 'message' => 'Done' ) );
}

add_action( 'wp_ajax_custom_action', 'handle_ajax_json' );

wp_send_json_success() sets the correct `Content-Type: application/json` header, encodes your data, and calls wp_die() internally. One function. No loose ends.

Use wp_send_json_error() for failures. Your frontend can check response.data.success consistently across all handlers.

AJAX responses should be clean and predictable. One missing line breaks everything silently.