Implementing Custom Transform Streams with _transform and _flush

A Transform stream is both readable and writable: it consumes input chunks, processes them, and pushes output chunks. It is the right tool whenever data must be mutated as it flows rather than buffered fully in memory.

• Writable side accepts data via write() / pipe() from upstream.
• Readable side emits processed data that downstream consumers read.

Typical backend uses: uppercasing/normalizing a payload, gzip-style encoding, CSV-to-JSON conversion, redacting secrets in a log pipeline, or counting bytes — all without loading the whole file or HTTP body into RAM.

A custom Transform stream is defined by implementing two internal methods. Node calls them for you — you never call them directly.

• _transform(chunk, encoding, callback) — invoked once per incoming chunk. Do your work, push() any output, then signal completion with callback().
• _flush(callback) — invoked once, after the last chunk, just before the stream ends. Use it to emit any trailing/aggregated data.

The leading underscore marks them as the framework-facing implementation. Consumers still use the public write, read, and pipe API.