How do you implement a custom stream Transform in Node.js?

Question

Accepted Answer

**Custom Transform** — modify data as it flows through a stream.

```javascript
const { Transform } = require("stream");

// JSON lines parser
class JSONLinesParser extends Transform {
    constructor(options) {
        super({ ...options, objectMode: true });
        this.buffer = "";
    }

_transform(chunk, encoding, callback) {
        this.buffer += chunk.toString();
        const lines = this.buffer.split("
");
        this.buffer = lines.pop();

for (const line of lines) {
            if (line.trim()) {
                try {
                    this.push(JSON.parse(line));
                } catch (err) {
                    this.emit("error", err);
                }
            }
        }
        callback();
    }

_flush(callback) {
        if (this.buffer.trim()) {
            try {
                this.push(JSON.parse(this.buffer));
            } catch (err) {
                this.emit("error", err);
            }
        }
        callback();
    }
}

// Usage
const { createReadStream } = require("fs");
const { pipeline } = require("stream/promises");

await pipeline(
    createReadStream("data.jsonl"),
    new JSONLinesParser(),
    new Writable({
        write(obj, encoding, callback) {
            console.log("Parsed:", obj);
            callback();
        },
    })
);
```

Node.js: How do you implement a custom stream Transform in Node.js?