Stream output to a local file

Deno Sandbox provides a sandboxed Linux microVM. This is useful for evaluating code that is not trusted or for testing code that is not safe to run in your main runtime.

You can stream output to a local file in a sandbox. This avoids buffering entire large artifacts in memory.

If you generate something sizable inside the sandbox (like big.txt below), you can pipe it out chunk-by-chunk over a ReadableStream, converting Node’s fs.WriteStream to a Web WritableStream for efficient transfer.

import { Sandbox } from "@deno/sandbox";
import fs from "node:fs";
import { Writable } from "node:stream";

await using sandbox = await Sandbox.create();

// Create a large file in the sandbox
await sandbox.fs.writeTextFile("big.txt", "#".repeat(5_000_000));

// Stream it out to a local file
const child = await sandbox.spawn("cat", {
  args: ["big.txt"],
  stdout: "piped",
});
const file = fs.createWriteStream("./big-local-copy.txt");
await child.stdout.pipeTo(Writable.toWeb(file));

const status = await child.status;
console.log("done:", status);

This pattern keeps memory usage flat, works well for logs or big binaries, and lets you persist sandbox results on the host without temporary files or stdout truncation.

For more information, see the Deno Sandbox documentation.