---
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-10904)
* [  TypeScript ](#tab-panel-10905)

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-10900)
* [  TypeScript ](#tab-panel-10901)

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-10898)
* [  TypeScript ](#tab-panel-10899)

JavaScript

```
// Browser JavaScriptconst 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 JavaScriptconst 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-10902)
* [  TypeScript ](#tab-panel-10903)

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-10906)
* [  TypeScript ](#tab-panel-10907)

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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