WooCommerce: Get order_id for existing order in checkout page

wc_order_id_in_checkout.php

<?php
/*
 * Context:
 * Sometime we may need to get the `order_id` in checkout page. When redirecting from `order-pay` you can grab it from `query_var`. But as soon as you reload the page it's lost.
 */
// You can access it from WooCommerce Session. For pending order there is a key named `order_awaiting_payment` in WooCommerce Session.

// Method 1: using `WC_Session_Handler`

$wc_session = new WC_Session_Handler();
$wc_session_data = $wc_session->get_session_data();

// Check if there is order_id or set it as 0
$order_id = isset($wc_session_data['order_awaiting_payment'])?$wc_session_data['order_awaiting_payment']:0;


// Method 2: accessing `WC_Session` directly using `WC()`

$order_id = WC()->session->get( 'order_awaiting_payment' );