---
title: Stream output
description: Handle real-time output from commands and processes.
image: https://developers.cloudflare.com/dev-products-preview.png
---

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# Stream output

This guide shows you how to handle real-time output from commands, processes, and code execution.

## When to use streaming

Use streaming when you need:

* **Real-time feedback** \- Show progress as it happens
* **Long-running operations** \- Builds, tests, installations that take time
* **Interactive applications** \- Chat bots, code execution, live demos
* **Large output** \- Process output incrementally instead of all at once
* **User experience** \- Prevent users from waiting with no feedback

Use non-streaming (`exec()`) for:

* **Quick operations** \- Commands that complete in seconds
* **Small output** \- When output fits easily in memory
* **Post-processing** \- When you need complete output before processing

## Stream command execution

Use `execStream()` to get real-time output:

* [  JavaScript ](#tab-panel-8155)
* [  TypeScript ](#tab-panel-8156)

JavaScript

```

import { getSandbox, parseSSEStream } from "@cloudflare/sandbox";


const sandbox = getSandbox(env.Sandbox, "my-sandbox");


const stream = await sandbox.execStream("npm run build");


for await (const event of parseSSEStream(stream)) {

  switch (event.type) {

    case "stdout":

      console.log(event.data);

      break;


    case "stderr":

      console.error(event.data);

      break;


    case "complete":

      console.log("Exit code:", event.exitCode);

      break;


    case "error":

      console.error("Failed:", event.error);

      break;

  }

}


```

TypeScript

```

import { getSandbox, parseSSEStream, type ExecEvent } from '@cloudflare/sandbox';


const sandbox = getSandbox(env.Sandbox, 'my-sandbox');


const stream = await sandbox.execStream('npm run build');


for await (const event of parseSSEStream<ExecEvent>(stream)) {

  switch (event.type) {

    case 'stdout':

      console.log(event.data);

      break;


    case 'stderr':

      console.error(event.data);

      break;


    case 'complete':

      console.log('Exit code:', event.exitCode);

      break;


    case 'error':

      console.error('Failed:', event.error);

      break;

  }

}


```

## Stream to client

Return streaming output to users via Server-Sent Events:

* [  JavaScript ](#tab-panel-8151)
* [  TypeScript ](#tab-panel-8152)

JavaScript

```

import { getSandbox } from "@cloudflare/sandbox";


export { Sandbox } from "@cloudflare/sandbox";


export default {

  async fetch(request, env) {

    const sandbox = getSandbox(env.Sandbox, "builder");


    const stream = await sandbox.execStream("npm run build");


    return new Response(stream, {

      headers: {

        "Content-Type": "text/event-stream",

        "Cache-Control": "no-cache",

      },

    });

  },

};


```

TypeScript

```

import { getSandbox } from '@cloudflare/sandbox';


export { Sandbox } from '@cloudflare/sandbox';


export default {

  async fetch(request: Request, env: Env): Promise<Response> {

    const sandbox = getSandbox(env.Sandbox, 'builder');


    const stream = await sandbox.execStream('npm run build');


    return new Response(stream, {

      headers: {

        'Content-Type': 'text/event-stream',

        'Cache-Control': 'no-cache'

      }

    });

  }

};


```

Client-side consumption:

* [  JavaScript ](#tab-panel-8149)
* [  TypeScript ](#tab-panel-8150)

JavaScript

```

// Browser JavaScript

const eventSource = new EventSource("/build");


eventSource.addEventListener("stdout", (event) => {

  const data = JSON.parse(event.data);

  console.log(data.data);

});


eventSource.addEventListener("complete", (event) => {

  const data = JSON.parse(event.data);

  console.log("Exit code:", data.exitCode);

  eventSource.close();

});


```

TypeScript

```

// Browser JavaScript

const eventSource = new EventSource('/build');


eventSource.addEventListener('stdout', (event) => {

  const data = JSON.parse(event.data);

  console.log(data.data);

});


eventSource.addEventListener('complete', (event) => {

  const data = JSON.parse(event.data);

  console.log('Exit code:', data.exitCode);

  eventSource.close();

});


```

## Stream process logs

Monitor background process output:

* [  JavaScript ](#tab-panel-8153)
* [  TypeScript ](#tab-panel-8154)

JavaScript

```

import { parseSSEStream } from "@cloudflare/sandbox";


const process = await sandbox.startProcess("node server.js");


const logStream = await sandbox.streamProcessLogs(process.id);


for await (const log of parseSSEStream(logStream)) {

  console.log(log.data);


  if (log.data.includes("Server listening")) {

    console.log("Server is ready");

    break;

  }

}


```

TypeScript

```

import { parseSSEStream, type LogEvent } from '@cloudflare/sandbox';


const process = await sandbox.startProcess('node server.js');


const logStream = await sandbox.streamProcessLogs(process.id);


for await (const log of parseSSEStream<LogEvent>(logStream)) {

  console.log(log.data);


  if (log.data.includes('Server listening')) {

    console.log('Server is ready');

    break;

  }

}


```

## Handle errors

Check exit codes and handle stream errors:

* [  JavaScript ](#tab-panel-8157)
* [  TypeScript ](#tab-panel-8158)

JavaScript

```

const stream = await sandbox.execStream("npm run build");


for await (const event of parseSSEStream(stream)) {

  switch (event.type) {

    case "stdout":

      console.log(event.data);

      break;


    case "error":

      throw new Error(`Build failed: ${event.error}`);


    case "complete":

      if (event.exitCode !== 0) {

        throw new Error(`Build failed with exit code ${event.exitCode}`);

      }

      break;

  }

}


```

TypeScript

```

const stream = await sandbox.execStream('npm run build');


for await (const event of parseSSEStream<ExecEvent>(stream)) {

  switch (event.type) {

    case 'stdout':

      console.log(event.data);

      break;


    case 'error':

      throw new Error(`Build failed: ${event.error}`);


    case 'complete':

      if (event.exitCode !== 0) {

        throw new Error(`Build failed with exit code ${event.exitCode}`);

      }

      break;

  }

}


```

## Best practices

* **Always consume streams** \- Don't let streams hang unconsumed
* **Handle all event types** \- Process stdout, stderr, complete, and error events
* **Check exit codes** \- Non-zero exit codes indicate failure
* **Provide feedback** \- Show progress to users for long operations

## Related resources

* [Commands API reference](https://developers.cloudflare.com/sandbox/api/commands/) \- Complete streaming API
* [Execute commands guide](https://developers.cloudflare.com/sandbox/guides/execute-commands/) \- Command execution patterns
* [Background processes guide](https://developers.cloudflare.com/sandbox/guides/background-processes/) \- Process log streaming
* [Code Interpreter guide](https://developers.cloudflare.com/sandbox/guides/code-execution/) \- Stream code execution output

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