WordPress Rest API endpoint to process payments via WC

woocommerce_payment_process_rest.php

<?php

/*
* Suitable for NON-HOSTED payment gateways wallets, funds, etc.
*
* You can also process HOSTED payment gateways like paypal, but you'll get a rediect URL at the end
*   and you have to handle that depending on what client (android, ios) you're catering to.
* You can have the redirect URL open up in a web-view-client and collect payment.
*
**/
function process_checkout($request)
    {
        $params = $request->get_json_params();

        // fields
        $_billing = array(
            'billing_first_name', 'billing_last_name', 'billing_city', 'billing_state',
            'billing_country', 'billing_address_1', 'billing_postcode', 'billing_phone', 'billing_email'
        );

        // only when ship_to_different_address is set to 1
        $_shipping = array(
            'shipping_first_name', 'shipping_last_name', 'shipping_address_1', 'shipping_city',
            'shipping_state', 'shipping_postcode'
        );

        // extra non-required fields
        $_extras = array(
            'billing_company', 'shipping_company', 'billing_address_2', 'shipping_address_2',
            'shipping_phone'
        );

        if( isset($params['ship_to_different_address']) && $params['ship_to_different_address'] == '1' ) {
            $intersect = array_diff( array_merge($_billing, $_shipping), array_keys($params));
        } else {
            $intersect = array_diff( $_billing, array_keys($params));
        }

        // CHECKS BEGINS
        if( ! empty($intersect) ) {
            jwt_auth_wp_json_error_data(
                __('Missing Billing / Shipping Details', 'jwt-rest-api'),
                array_values($intersect)
            );
        }

        // TODO:  check for valid payment/ shipping method
        if(! isset($params['payment_method']) ) {
            jwt_auth_wp_json_error_data(
                __('Payment Method Missing / Invalid', 'jwt-rest-api'),
                array_values($intersect)
            );
        }

        else if(! isset($params['shipping_method']) ) {
            jwt_auth_wp_json_error_data(
                __('Shipping Method Missing / Invalid', 'jwt-rest-api'),
                array_values($intersect)
            );
        }

        else if(! isset($params['terms']) || $params['terms'] != '1' ) {
            jwt_auth_wp_json_error_data(
                __('Please read and accept the terms and conditions to proceed with your order.', 'jwt-rest-api'),
                array_values($intersect)
            );
        }

        else if( isset($params['createaccount']) && (!isset($params['account_password']) || empty($params['account_password'])) ) {
            jwt_auth_wp_json_error_data(
                __('Please provide a password if you want to create an account.', 'jwt-rest-api'),
                array_values($intersect)
            );
        }
        // CHECKS PASSED

        // start building post global var for checkout
        $_POST    = array();

        $postvars =
            isset($params['ship_to_different_address']) ?
            array_merge($_billing, $_shipping) :
            $_billing;

        foreach ( $postvars as $value ) {
            $_POST[ $value ] = sanitize_text_field( $params[$value] );
        }

        foreach ( $_extras as $value ) {
            if( isset($params[$value]) ) {
                $_POST[ $value ] = sanitize_text_field( $params[$value] );
            }
        }

        // $_POST['woocommerce_checkout_update_totals'] = '1';

        $_POST['terms']           = '1';
        $_POST['payment_method']  = $payment_method = $params['payment_method'];
        $_POST['shipping_method'] = $params['shipping_method'];

        if( isset($params['createaccount']) ) {
            $_POST['account_password'] =  $params['account_password'];
        }

        // create a nonce & verify at the same request; otherwise it'll fail the payment
        $_REQUEST['_wpnonce'] =
            wp_create_nonce( 'woocommerce-process_checkout' );

        // clear all accidental uncleard notices; otherwise the payment might fail
        wc_clear_notices();

        // make it look like an ajax request; so that it won't accidently redirect anywhere  
        wc_maybe_define_constant( 'DOING_AJAX', 1 );

        // for orders that need payments
        add_filter('woocommerce_payment_successful_result', function($data) use($payment_method) {
        	// handle checkout response however you want wp_send_json()
            
            // it may have a redirect ($data['redirect']) url if you processed checkout using Hosted PG.
            // you can handle depending on what your client is [ mobile app - android, ios ], open this redirect url in
            // a webview maybe
        }, 99999);

        // for orders that don't need any payment - store pick-up, cash on delivery, etc.
        //  or maybe when the entire order is paid using some sort of funds and order total turns to zero
        add_filter('woocommerce_checkout_no_payment_needed_redirect', function($data) use($payment_method) {
            // handle checkout response however you want wp_send_json()
        });

        WC()->checkout()->process_checkout();
    }