Use Koa to stream an ExcelJS workbook immediately to the browser - node.js

Question

What I would like is to stream my ExcelJS workbook little by little to the browser. The problem is that I'm very uncomfortable with streams.
The facts:
I'm on Node JS v.14
I'm using last version of ExcelJS library to generate a big Excel file
I'm using last version of Koa as a REST API tool
Here's a simplified part of my code. The Excel workbook is ok. If I create a full buffer in memory and send this buffer to Koa, I can download it. But when I'm trying to stream and pass a stream to koa, things go wrong. The way of passing stream to ExcelJS is the right way, from their docs.
EDIT
Here's a full working work base (except for streams).
package.json
{
"name": "stackoverflow-exceljs-koa",
"private": true,
"version": "0.0.1",
"author": "rekam",
"dependencies": {
"exceljs": "^4.3.0",
"koa": "^2.13.4"
}
}
index.js
const Koa = require('koa');
const app = new Koa();
const { exportExcelData } = require('./builder');
app.use(async ctx => {
if (ctx.request.url === '/file') {
await exportExcelData('useFile', './test.xlsx');
ctx.body = 'Done, file saved at ' + __dirname + '/test.xlsx';
return;
}
if (ctx.request.url === '/stream') {
// const stream = new Stream();
// ctx.body = stream;
// await exportExcelData('useStream', stream);
ctx.body = 'to be implemented';
return;
}
ctx.body = 'Not Found';
ctx.status = 404;
});
app.listen(8101);
console.log('visit http://localhost:8101/file or /stream');
builder.js
const Excel = require('exceljs');
module.exports = {
async exportExcelData(type, streamOrFilename) {
const limit = 100;
let offset = 0;
const workbook = new Excel.stream.xlsx.WorkbookWriter({
stream: type === 'useStream' ? streamOrFilename : undefined,
filename: type === 'useFile' ? streamOrFilename : undefined,
useSharedStrings: true,
useStyles: true
});
const sheet = workbook.addWorksheet('Export');
sheet.columns = [{
header: 'ID',
key: 'id'
}];
const build = async (offset) => {
const rows = Array.from({ length: limit }).map((_, i) => ({ id: i + offset }));
if (offset > 10000) {
rows.pop(); // will break while looop
}
rows.forEach(row => sheet.addRow(row).commit());
return rows;
};
let rows;
do {
rows = await build(offset);
offset += limit;
} while (rows.length === limit);
sheet.commit();
await workbook.commit();
return;
}
};
usage
$ node index.js