Example of my code. Worker for XLSX and func for it

gistfile1.txt

import XLSX from 'xlsx';

const getWorkbook = (fileListItem, index) =>
  new Promise(resolve => {
    const reader = new FileReader();
    reader.readAsBinaryString(fileListItem.file);

    reader.onload = () => {
      const wb = XLSX.read(reader.result, { sheetRows: 50, type: 'binary' });
      console.log(wb)
      resolve({ index, wb, wbName: fileListItem.file.name });
    };

    reader.onerror = error => {
      console.log('Error: ', error);
    };
  });

self.onmessage = ({ data }) => {
  getWorkbook(data.item, data.index)
    .then(result => {
      if (typeof result.index !== 'undefined') {
        self.postMessage({ index: result.index, wb: result.wb });
      }
    })
}

-----------------------------------------

  useEffect(() => {
    fileListItems.map((item, index) => {
      if (!item.workbook) {
        const worker = new Worker(new URL('./worker.js', import.meta.url));
        worker.postMessage({item, index});
        worker.onmessage = ({ data }) => setFileWorkbook(data.index, data.wb);
      }
    });
  }, [fileListItems]);