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## Resource: wix-window-frontend

## Namespace: warmup-data

## Article: Introduction

## Article Link: https://dev.wix.com/docs/velo/apis/wix-window-frontend/warmup-data/introduction.md

## Article Content:

# Introduction
To improve performance the [rendering process](https://dev.wix.com/docs/develop-websites/articles/coding-with-velo/frontend-code/page-rendering/about-page-rendering.md) 
 is split in 2 when possible: once on the server and once in the browser. When that's not 
 possible, all rendering happens in the browser.

 Because rendering can occur twice, 
 it's preferred to perform time-consuming tasks only once, such as querying 
 collections or working with the results of network requests from external sites. 
 If possible, it's best to perform these operations on the server. 

 Requesting the warmup data from the server returns `undefined` in the following situations:
 + When browser rendering happens before server-side rendering.
 + When there is only browser rendering, for example in case visitors navigate from page to page within a site.

 The page code must check for these scenarios and provide a fallback alternative for getting the data.

 Use the Warmup Data API and `env` property  of the Rendering API
 to track when and where your code is running. This lets you selectively run parts of your code.

 > **Notes:**
 > + Rendering never occurs server-side when previewing a site.
 > + Backend rendering only occurs when site visitors initially enter a site, and not when sitevisitors navigate from page-to-page within a site.

 To use the Warmup Data API, import `warmupData` from the `wix-window-frontend` module:

   ```javascript
   import { warmupData } from 'wix-window-frontend';
   ```