Firebase : Cloud functions write to local storage from multi part

busboy is the nam package that can be used for this purpose. Below is the code snippet for it.

exports.uploadFile = functions.https.onRequest((req, res) => {
  if (req.method === 'POST') {
    const busboy = new Busboy({headers: req.headers});
    const tmpdir = os.tmpdir();

    // This object will accumulate all the fields, keyed by their name
    const fields = {};

    // This object will accumulate all the uploaded files, keyed by their name.
    const uploads = {};

    // This code will process each non-file field in the form.
    busboy.on('field', (fieldname, val) => {
      // TODO(developer): Process submitted field values here
      console.log(`Processed field ${fieldname}: ${val}.`);
      fields[fieldname] = val;
    });

    let fileWrites = [];

    // This code will process each file uploaded.
    busboy.on('file', (fieldname, file, filename) => {
      // Note: os.tmpdir() points to an in-memory file system on GCF
      // Thus, any files in it must fit in the instance's memory.
      console.log(`Processed file ${filename}`);
      const filepath = path.join(tmpdir, filename);
      uploads[fieldname] = filepath;
      console.log(`Processed full file path ${filepath}`);

      const writeStream = fs.createWriteStream(filepath);
      file.pipe(writeStream);

      // File was processed by Busboy; wait for it to be written to disk.
      const promise = new Promise((resolve, reject) => {
        file.on('end', () => {
          writeStream.end();
        });
        writeStream.on('finish', resolve);
        writeStream.on('error', reject);
      });
      fileWrites.push(promise);
    });

    // Triggered once all uploaded files are processed by Busboy.
    // We still need to wait for the disk writes (saves) to complete.
    busboy.on('finish', () => {
      Promise.all(fileWrites).then(() => {
        // TODO(developer): Process saved files here
        for (const name in uploads) {
          const file = uploads[name];
          fs.unlinkSync(file);
        }
        res.send();
      });
    });

    busboy.end(req.rawBody);
  } else {
    // Return a "method not allowed" error
    res.status(405).end();
  }
});

reference:
https://cloud.google.com/functions/docs/writing/http#multipart_data_and_file_uploads