Get null data response when use stream's event listeners nodejs

Question

I have problem when try to calculate size of stream file with nestjs and fast-csv lib. This is file service:

import { format } from 'fast-csv';

async export() {
    const stream = format({ headers: true });

    stream.write(['id', 'first_name', 'last_name', 'is_active']);

    let size = 0;

    // There is a problem when I add event listener to calculate stream size 
    // stream.on('data', (chunk) => {
    //   size += chunk.length;
    // });

    // stream.on('end', () => {
    //   console.log(size);
    // });

    await this.getData(stream)().finally(() => {
      stream.end();
    });

    return stream ? { stream, size } : null;
  }

  private getData = (stream: Transform) => async () => {
    for (let i = 0; i < 1; i++) {
      const users = await User.find();

      if (users.length === 0) {
        break;
      }

      for (let i = 0; i < users.length; i++) {
        const row = users[i];

        stream.write([row.id, row.firstName, row.lastName, row.isActive]);
      }
    }
  };

This is file controller:

@Post('export')
  async exporrt() {
    const data = await this.appService.export();

    console.log('size', data.size);

    return new StreamableFile(data.stream, {
      disposition: `attachment; filename=test.csv`,
      type: 'text/csv; charset=utf-8',
    });
  }

The code run well and below is the result when I call API export

However when I try to calculate stream size with stream's event listener, I can get the size of stream but I got no data in response:

How can I calculate stream size with on('data') or with other solution. Thank for your attention.