Ttdfjt

You need to modify the successAction() in

/app/code/core/Mage/Checkout/controllers/OnepageController.php

Modified action would like this

public function successAction()
 
 /*
 $session = $this->getOnepage()->getCheckout();
 if (!$session->getLastSuccessQuoteId()) 
 $this->_redirect('checkout/cart');
 return;
 

 $lastQuoteId = $session->getLastQuoteId();
 $lastOrderId = $session->getLastOrderId();
 $lastRecurringProfiles = $session->getLastRecurringProfileIds();
 if (!$lastQuoteId 

Hope I'm not too self-promoting, but I created a free extension that can be quickly installed in Magento, and allows you to preview the order success-page for any order - simply by accessing an URL:
http://www.yireo.com/software/magento-extensions/checkout-tester

I think it's better to just comment $session->clear(); and add products manually, that worked for me but commenting the whole action gave me syntax errors.

For Magento 2:

If you want to style or customize in success page after order success page redirects to cart page.

Now solution is here:

Go To vendor/magento/module-checkout/Controller/Onepage
open Success.php File.

In this file, you see Below Code

 if (!$this->_objectManager->get('MagentoCheckoutModelSessionSuccessValidator')->isValid()) 
 return $this->resultRedirectFactory->create()->setPath('checkout/cart');
 

Just comment on this code and your problem is solved. After comment this you don't redirect to cart page.

Instead of sending out emails from local/development copy you can dump the content of the email to a file and then just see locally, which in my opinion will be really handy. Here's how that can be acheived. First all the emails are sent from

Mage_Core_Model_Email_Template::send($email, $name = null, array $variables = array())

Find that, and add the following lines.

 $this->setUseAbsoluteLinks(true);
 $text = $this->getProcessedTemplate($variables, true);

 if($this->isPlain()) 
 $mail->setBodyText($text);
 else 
 $mail->setBodyHTML($text);
 
 // Added Code //
 $filePath = Mage::getBaseDir() . DS . 'email.html';
 Mage::log($filePath);
 file_put_contents($filePath, $text);
 // Added Code Ends //

 $mail->setSubject('=?utf-8?B?' . base64_encode($this->getProcessedTemplateSubject($variables)) . '?=');
 $mail->setFrom($this->getSenderEmail(), $this->getSenderName());

After this after you create any order an email.html will be created in the Magento document root and you can open that in browser to see the output.

Next, to send/re-send any order emails you can just log in admin and for each order there is Send Email button which will trigger this script and you can see the newly changed template written in the same file. I think this is one of the best way to see order or any other emails.

Please note to remove the added code when you are done.

You have to update code:

/app/code/core/Mage/Checkout/controllers/OnepageController.php

Update function:

public function successAction()

 $session = $this->getOnepage()->getCheckout();
 if (!$session->getLastSuccessQuoteId()) 
 $this->_redirect('checkout/cart');
 return;
 

 $lastQuoteId = $session->getLastQuoteId();
 $lastOrderId = $session->getLastOrderId();
 $lastRecurringProfiles = $session->getLastRecurringProfileIds();
 if (!$lastQuoteId 

Just comment:

//$session->clear();

Magento 2

As another answer notes, you can comment out the redirect in MagentoCheckoutControllerOnepageSuccess::execute and force the checkout success page to load. But when the block, MagentoCheckoutBlockOnepageSuccess, loads no order data will be present because MagentoCheckoutModelSession::getLastRealOrder won't return an order. A better options would be an after interceptor on the controller class's execute method, where you can set a value for lastRealOrderId in the checkout session.

This approach also provides an opportunity to dispatch the checkout_onepage_controller_success_action event with your desired order. The event will trigger the observers MagentoGoogleAdwordsObserverSetConversionValueObserver and MagentoGoogleAnalyticsObserverSetGoogleAnalyticsOnOrderSuccessPageViewObserver with your test data.

The following is a fairly basic module creating the interceptor described above and allows setting the order by appending an order query parameter with the desired increment id to the success page url. It currently won't work the the multishipping success route. It can be downloaded on github: https://github.com/pmclain/magento2-successtest

app/code/Pmclain/SuccessTest/etc/frontend/di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
 <type name="MagentoCheckoutControllerOnepageSuccess">
 <plugin name="pmclain_successtest_checkout_controller_onepage_success"
 type="PmclainSuccessTestPluginSuccess"
 sortOrder="10" />
 </type>
</config>

app/code/Pmclain/SuccessTest/Plugin/Success.php

<?php
namespace PmclainSuccessTestPlugin;

use MagentoFrameworkEventManagerInterface;
use MagentoFrameworkViewResultPageFactory;
use MagentoFrameworkAppConfigScopeConfigInterface;
use MagentoStoreModelScopeInterface;
use MagentoCheckoutModelSession;
use MagentoSalesModelOrderFactory;
use MagentoSalesModelOrder;

class Success

 /** @var ManagerInterface */
 protected $_eventManager;

 /** @var PageFactory */
 protected $_resultPageFactory;

 /** @var ScopeConfigInterface */
 protected $_scopeConfig;

 /** @var OrderFactory */
 protected $_orderFactory;

 /** @var Order */
 protected $_order;

 /** @var Session */
 protected $_checkoutSession;

 /**
 * Success constructor.
 * @param ManagerInterface $eventManager
 * @param PageFactory $resultPageFactory
 * @param ScopeConfigInterface $scopeConfig
 * @param OrderFactory $orderFactory
 * @param Session $session
 */
 public function __construct(
 ManagerInterface $eventManager,
 PageFactory $resultPageFactory,
 ScopeConfigInterface $scopeConfig,
 OrderFactory $orderFactory,
 Session $session
 ) 
 $this->_eventManager = $eventManager;
 $this->_resultPageFactory = $resultPageFactory;
 $this->_scopeConfig = $scopeConfig;
 $this->_orderFactory = $orderFactory;
 $this->_checkoutSession = $session;
 

 /**
 * @param MagentoCheckoutControllerOnepageSuccess $subject
 * @param $result
 * @return MagentoFrameworkViewResultPage
 */
 public function afterExecute(MagentoCheckoutControllerOnepageSuccess $subject, $result)
 
 if (!$this->_isEnabled()) 
 return $result;
 

 $order = $this->_getTestOrder($subject->getRequest()->getParam('order'));

 if (!$order->getId()) 
 return $result;
 

 $this->_checkoutSession->setLastRealOrderId($order->getIncrementId());

 $resultPage = $this->_resultPageFactory->create();

 $this->_eventManager->dispatch(
 'checkout_onepage_controller_success_action',
 ['order_ids' => [$order->getId()]]
 );

 return $resultPage;
 

 /**
 * @return bool
 */
 protected function _isEnabled()
 
 if ($this->_scopeConfig->getValue('dev/debug/success_test', ScopeInterface::SCOPE_STORE)) 
 return true;
 

 return false;
 

 /**
 * @param $incrementId string

app/code/Pmclain/SuccessTest/etc/adminhtml/system.xml
Below adds admin panel options to enable/disable the interceptor.

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
 <system>
 <section id="dev">
 <group id="debug">
 <field id="success_test" translate="label" type="select" sortOrder="30" showInDefault="1" showInWebsite="1" showInStore="0">
 <label>Enable Checkout Success Page Testing</label>
 <source_model>MagentoConfigModelConfigSourceYesno</source_model>
 </field>
 </group>
 </section>
 </system>
</config>

app/code/Pmclain/SuccessTest/etc/config.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
 <default>
 <dev>
 <debug>
 <success_test>0</success_test>
 </debug>
 </dev>
 </default>
</config>

app/code/Pmclain/SuccessTest/etc/module.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
 <module name="Pmclain_SuccessTest" setup_version="0.0.1">
 <sequence>
 <module name="Magento_Backend" />
 <module name="Magento_Checkout" />
 </sequence>
 </module>
</config>

app/code/Pmclain/SuccessTest/resgistration.php

<?php
MagentoFrameworkComponentComponentRegistrar::register(
 MagentoFrameworkComponentComponentRegistrar::MODULE,
 'Pmclain_SuccessTest',
 __DIR__
);