https://impler.io/wp-content/themes/neve-child-master/js/converter-files/xlsx-to-csv-worker.js

1/* this callback will run once the main context sends a message */
2self.addEventListener('message', (e) => {
3    importScripts("https://cdn.sheetjs.com/xlsx-latest/package/dist/xlsx.full.min.js");
4    var reader = new FileReader();
5    reader.onload = evt => {
6        const bstr = evt.target.result;
7        const wb = XLSX.read(bstr, {type: "binary"});
8        if(wb.SheetNames.length > 1) {
9            const files = [];
10            wb.SheetNames.forEach(sheetName => {
11                const response = XLSX.utils.sheet_to_csv(wb.Sheets[sheetName], {
12                    FS: ',',
13                    RS: '\n',
14                    strip: true,
15                    blankrows: false,
16                });
17                files.push(response);
18            });
19
20            /* Pass the result data back */
21            postMessage({ files, sheetNames: wb.SheetNames });
22        } else {
23            const response = XLSX.utils.sheet_to_csv(wb.Sheets[wb.SheetNames[0]], {
24                FS: ',',
25                RS: '\n',
26                strip: true,
27                blankrows: false,
28            });
29            /* Pass the result data back */
30            postMessage({ file: response });
31        }
32    }
33    reader.readAsArrayBuffer(e.data);
34}, false);