Node.js: Why is the transform function of a Transform stream never getting called?

Question

I am trying to write a custom transformer stream in Node.js 12. Specifically I take in json objects in a stream (Database driver) and return an object transformed. But my transformer functions never get called. I have also tried this by overriding the streams.Transform class.

I want to make the custom transform generic so I am enclosing it in a closure, in order to pass in a generic function:

// transformStream.js
var through2, transformStream;

through2 = require('through2');

transformStream = (handler) => {
  // Through2 in Object Mode
  _transformStream = through2.obj((data, encoding, callback) => {
    console.log(data); // Never called
    this.push(handler(data));
    return callback();
    // also tried:
    // return callback(null, handler(data));
  });

  return _transformStream;
};

module.exports = transformStream;

Here is the test rig to try it out:

// transformStream.test.js
var jsonStream, through2, transformFunc, transformStream, transformer;

through2 = require('through2');
transformStream = require('./transformStream.js');

// Convert back to a string buffer for console output.
jsonStream = through2.obj(function(chunk, encoding, callback) {
  return callback(null, JSON.stringify(chunk, null, 2) + '\n');
});

transformFunc = function(data) {
  console.log("called with data", data); // Never called!
  data.c = data.a * data.b;
  return data;
};

// deviceStream.pipe(process.stdout)
transformer = transformStream(transformFunc);

transformer.on("error", function(error) {
  return console.error(`Error in Transform: ${error.message}`);
});

transformer.pipe(jsonStream).pipe(process.stdout);

transformer.push({
  a: 1,
  b: 2
});

The stream appears to work, never calls the actual transform code, and always returns just the original json:

{
  A: 1,
  b: 2
}

in the console.

I expect to see: { a: 1, b:2, c:2 }

EDIT: I also have another version using classes (bypassing through2) with the same exact issue:

module.exports = TransformStream = class TransformStream extends Transform {
  constructor(handler, {debug, highWaterMark, ...options}) {
    super({
      highWaterMark: highWaterMark || 10,
      autoDestroy: true,
      emitClose: true,
      objectMode: true,
      debug: true
    });
    this._transform = this._transform.bind(this);
    this.handler = handler;
    this.debug = debug;
    this.options = options;
  }
};

TransformStream.prototype._transform = (data, encoding, callback) => {
  if (this.debug) {
    console.log(data);
  }
  return callback(null, this.handler(data));
};

Answer 1

Apparently doing a TransformClass.push calls the internal output buffer for the stream. The stream is actually expecting a .write({}) method, and appropriately calls the _transform functions.

In the test rig the final test should be:

transformer.write({
  a: 1,
  b: 2
});