What are streams in Node.js and when should you use them?

Question

Accepted Answer

**Streams** process data **incrementally, in chunks**, rather than loading it all into memory at once. This makes them essential for large data (big files, network transfers) where buffering everything would exhaust memory.

## The problem streams solve

```js
// ❌ loads the ENTIRE file into memory — crashes on a 10GB file
const data = await fs.promises.readFile("huge.csv");
process(data);

// ✅ stream it — constant memory, processes chunk by chunk
fs.createReadStream("huge.csv")
  .pipe(transform)
  .pipe(fs.createWriteStream("out.csv"));
```

With streams, memory usage stays flat regardless of file size — you handle one chunk at a time.

## The four stream types

```text
Readable  → source you read FROM (fs.createReadStream, http request)
Writable  → destination you write TO (fs.createWriteStream, http response)
Duplex    → both readable & writable (TCP socket)
Transform → modify data as it passes through (zlib gzip, encryption)
```

## pipe — connect streams

```js
import { createReadStream, createWriteStream } from "fs";
import { createGzip } from "zlib";

// read → compress → write, all streaming, low memory
createReadStream("file.txt")
  .pipe(createGzip())            // Transform: compress on the fly
  .pipe(createWriteStream("file.txt.gz"));
```

`pipe` wires a readable to a writable (with transforms in between), automatically managing the flow — the idiomatic way to compose streams.

## pipeline — pipe with proper error handling

```js
import { pipeline } from "stream/promises";

await pipeline(
  createReadStream("in.txt"),
  createGzip(),
  createWriteStream("out.gz")
); // ✅ cleans up all streams on error/completion (pipe alone leaks on errors)
```

`pipeline` is preferred over chained `.pipe()` because it propagates errors and cleans up every stream properly.

## Where streams appear

```text
✓ HTTP request/response bodies (req and res ARE streams)
✓ File reading/writing, file uploads/downloads
✓ Compression (zlib), encryption (crypto)
✓ Database cursors, large query results
✓ Real-time data processing, video streaming
```

## When NOT to bother

```text
Small data that fits comfortably in memory → readFile/simple buffering is simpler.
Streams add complexity; use them for LARGE or continuous data.
```

## Why it matters

Streams are a defining Node feature for **memory-efficient** handling of large or continuous data — the difference between an app that crashes on a big upload and one that handles any size with flat memory.

Understanding the four types, `pipe`/`pipeline` composition (and why `pipeline` is safer), and that HTTP requests/responses are themselves streams is essential for building scalable file-handling, data-processing, and proxying services in Node.