Please explain how these hooks work

Question

I have read about hooks and actions and I know that when we see do_action('some_hook') it means that somewhere add_action('some_hook', function() {...}); or function some_hook() exists in a theme's functions.php.

But what is it?! :

From woocommerce/includes/class-wc-checkout.php line #943:

public function process_checkout() {
    try {
        /////////////
        do_action( 'woocommerce_before_checkout_process' ); <----!!!

        if ( WC()->cart->is_empty() ) {
            throw new Exception( sprintf( __( 'Sorry, your session has expired. <a href="%s" class="wc-backward">Return to shop</a>', 'woocommerce' ), esc_url( wc_get_page_permalink( 'shop' ) ) ) );
        }

        do_action( 'woocommerce_checkout_process' ); <-----!!!

I can't find woocommerce_checkout_process and woocommerce_before_checkout_process anywhere in files! What do they do?

Answer 1

It is possible for action hooks to be provided without being used, which is what's happening here.

woocommerce_before_checkout_process and woocommerce_checkout_process are hooks provided by WooCommerce, but WooCommerce does not itself attach callback functions to either of these hooks. They are provided to allow plugins and themes to run code at the time that the respective hooks are triggered.

If you'd like to attach a function to the woocommerce_checkout_process hook (for example), you'd add the following code to your theme or plugin:

add_action( 'woocommerce_checkout_process', 'wpse_woocommerce_checkout_process' );
function wpse_woocommerce_checkout_process() {
    // Do something...
}

This code will run when the line calling do_action( 'woocommerce_checkout_process' ); in WooCommerce is executed.