nocache_headers() │ WP 2.0.0

Suppose we need our AJAX request to be cached in the browser, and upon repeating this request within a specified time, the browser should take the data from the cache and not make an AJAX request.

This can be achieved by setting the necessary headers for caching. This can be done directly or through the filter nocache_headers.

Option: through the nocache_headers filter

The filter needs to be inserted directly into the PHP code, and in the filter, check that our request is indeed triggered, so that caching works for other requests.

Suppose our AJAX event is called my_ajax_action, then:

add_filter( 'nocache_headers', function( $headers ){

	// enable caching only for our event
	if( $_REQUEST['action'] === 'my_ajax_action' ){
		$life_time = HOUR_IN_SECONDS; // cache for one hour

		$headers = array(
			'Expires'       => gmdate( 'D, d M Y H:i:s', time() + $life_time ) .' GMT',
			'Cache-Control' => 'public',
		);
	}

	return $headers;
});

// AJAX request handler function
add_action( 'wp_ajax_'.'my_ajax_action', 'my_ajax_action_function' );
function my_ajax_action_function(){
	// do something and return the result for the AJAX request
	// the result of this request will be cached in the browser!
}

The advantage of this option:
Caching can be set up immediately for several events by simply adding a check for the event name.

Option: directly in the code

Directly in the code is often more convenient because headers can be set right in the AJAX request handler function.

Suppose our AJAX event is called my_ajax_action, then:

// AJAX request handler function
add_action( 'wp_ajax_'.'my_ajax_action', 'my_ajax_action_function' );
function my_ajax_action_function(){

	// enable caching in the browser
	$life_time = HOUR_IN_SECONDS; // cache for one hour
	header( 'Expires: ' . gmdate('D, d M Y H:i:s', time() + $life_time) . ' GMT' );
	header( 'Cache-control: public' );

	// do something and return the result for the AJAX request
	// the result of this request will be cached in the browser!
}

The advantage of this option:
Less code and caching can be set up right in the handler function.

Cancel page caching before executing any code related to sessions or that outputs information that should always be up-to-date:

// set the headers overriding the browser cache
nocache_headers();

// execute the code

Let's add our additional headers for all calls to the function wp_get_nocache_headers(). To do this, we use the hook nocache_headers:

add_filter( 'nocache_headers', function( $headers ) {

	return [
		'Cache-Control' => 'no-store, no-cache, must-revalidate, max-age=0, some-custom-thing',
		'Pragma'        => 'no-cache',
		'Expires'       => gmdate( 'D, d M Y H:i:s \G\M\T', time() )
	] + $headers;
} );