---
title: "Start a JSS app in connected mode"
slug: "start-a-jss-app-in-connected-mode"
abstract: "Develop locally against a running Sitecore instance"
---
# Start a JSS app in connected mode

To develop a JSS application locally against a running Sitecore instance, you must start your JSS application in connected mode.

When running your JSS application in connected mode, the rendering of the application is performed by the browser.

The Sitecore databases hold the content, layout data, and component registrations, so you do not need to define mock data locally in the application. The application retrieves data from Sitecore through HTTP requests.

The following diagram shows how the HTTP requests to acquire Sitecore data.

![HTTP data requests from a JSS app in connected mode.](https://doc.sitecore.com/api/markdown-image/xp/en/developers/hd/20/sitecore-headless-development/images/ApplicationModesConnected16_.svg)

> [!tip]
> JSS applications include a script in their `package.json` file to start the application in connected mode.

To start the application in connected mode:

1. Follow the steps to [connect and deploy your application to Sitecore](https://doc.sitecore.com/xp/en/developers/hd/20/sitecore-headless-development/walkthrough--connecting-a-jss-application-to-sitecore.html "Walkthrough: Connecting a JSS application to Sitecore").
2. In a terminal, run the following script:

    ```shell
    jss start:connected
    ```

    The script starts the local development server.

    > [!note]
    > If you [migrated from the code-first to the Sitecore-first](https://doc.sitecore.com/xp/en/developers/hd/20/sitecore-headless-development/transitioning-from-code-first-to-sitecore-first.html "Transitioning from code-first to Sitecore-first") development workflow and renamed the script `jss start:connected`, you must run the newly named script instead.

3. In your browser, open `http://localhost:3000` if it does not open automatically.

> [!note]
> If you are signed in to Sitecore in Experience Editor mode, running the JSS app in connected mode might result in extra code-like markup being delivered to your JSS application. If this happens, switch your site to Preview or Normal mode in Sitecore and the extra markup disappears.