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How do you implement a custom Transform stream?

Q: How do you implement a custom Transform stream?

**TL;DR.** A **Transform** stream reads input, transforms each chunk, and writes output — it's a Duplex you customize by implementing _transform(chunk, encoding, callback) (push processed data) and optionally _flush(callback) (emit trailing output). Drop it into a pipeline() for streaming parsing, encryption, or line processing. chunk in _transform() map / push <path class='d-edge-accent' d='M304 74 H352' marker-end='url(#f5

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Understand the problem

Subclassing Transform to process data chunk-by-chunk.

nodejsstreamstransformbackpressure

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Uppercase Transform

import { Transform } from 'node:stream';

const upper = new Transform({
  transform(chunk, _enc, cb) {
    this.push(chunk.toString().toUpperCase()); // process + emit
    cb();                                       // ready for next (backpressure)
  },
});

process.stdin.pipe(upper).pipe(process.stdout);

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