> ## Documentation Index
> Fetch the complete documentation index at: https://docs.kvant.sbs/jeston/llms.txt
> Use this file to discover all available pages before exploring further.

# Build your first Jeston application

# Build your first Jeston application

***

## Start · First application

> **Purpose** — Connect a page, a server handler, and a real request contract into one small vertical slice.

A first application whose behavior can be inspected from browser to server response.

### Key concepts

<Columns cols={3}>
  <Card title="Page" icon="sparkles">
    Initial HTML
  </Card>

  <Card title="API" icon="shield-check">
    Status and JSON
  </Card>

  <Card title="Failure" icon="gauge-high">
    Safe error body
  </Card>
</Columns>

### What to verify

| Area    | Expected state  | Evidence                                |
| ------- | --------------- | --------------------------------------- |
| Page    | Initial HTML    | Useful without waiting for client work. |
| API     | Status and JSON | Stable for callers and tests.           |
| Failure | Safe error body | Actionable without leaking internals.   |

### Working example

```text theme={null}
request -> Jeston runtime -> typed boundary -> observable response
```

### Before you move on

A vertical slice exposes integration mistakes earlier than a collection of disconnected components.

### Implementation notes

| Engineering move              | Guidance                                                  |
| ----------------------------- | --------------------------------------------------------- |
| **Create the route**          | Give the page or API endpoint a narrow responsibility.    |
| **Validate at the edge**      | Reject malformed input before domain work or persistence. |
| **Return an explicit result** | Choose status, headers, and body intentionally.           |
| **Exercise the failure path** | Test bad input, missing data, and a cancelled request.    |

### Decision lens

| Mode     | Practical emphasis                                             |
| -------- | -------------------------------------------------------------- |
| **Page** | Use SSR for initial content and clear loading boundaries.      |
| **API**  | Keep method handling and response shapes explicit.             |
| **Test** | Assert observable behavior rather than implementation details. |

## Related topics

<Columns cols={3}>
  * [server · **Understand SSR**](/jeston/jeston/core/react-ssr) — Read the focused guide for this boundary.
  * [brackets-curly · **Define API behavior**](/jeston/jeston/core/api-routes) — Read the focused guide for this boundary.
  * [globe · **Read HTTP contracts**](/jeston/jeston/reference/http-contracts) — Read the focused guide for this boundary.
</Columns>

## References

[1]: https://github.com/jeffersoncampos12p-dev/jeston "Jeston source repository"

[2]: https://www.npmjs.com/package/@kvantjs/jeston "Jeston package on npm"

[3]: https://nodejs.org/api/http.html "Node.js HTTP API"

[4]: https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal "AbortSignal Web API"

[5]: https://react.dev/reference/react-dom/server "React server rendering APIs"

[6]: https://www.typescriptlang.org/docs/handbook/intro.html "TypeScript handbook"