modern-xlsx works in any environment with WASM support:

• Node.js 24+ (uses node:fs for file operations)
• Bun (full support)
• Deno (full support)
• Browsers (Chrome, Firefox, Safari, Edge — all modern versions)

The TypeScript source uses modern features available in Node.js 24+. If you need older Node.js support, you can try building from source with appropriate transpilation, but this is not officially supported.

No. modern-xlsx only supports the OOXML .xlsx format (Office 2007+). The legacy .xls binary format is not supported.

Yes. v1.0.0 is the first stable release with 1,711 tests (424 Rust + 1,287 TypeScript) covering reading, writing, styling, encryption, charts, pivot tables, formula evaluation, and round-trip fidelity. The API is frozen and production-ready.

const input = document.querySelector('input[type="file"]');
input.addEventListener('change', async (e) => {
 const file = e.target.files[0];
 const buffer = new Uint8Array(await file.arrayBuffer());
 const wb = await readBuffer(buffer);
 console.log(wb.sheetNames);
});

The reader resolves shared strings automatically. Cell values are returned as their actual text content — you don't need to look up SST indices manually.

Yes! As of v0.6.0, modern-xlsx supports both reading and writing encrypted XLSX files:

// Read encrypted file
const wb = await readFile('protected.xlsx', { password: 'mypassword' });
// Write encrypted file
await wb.toFile('output.xlsx', { password: 'newpassword' });

Supports Agile Encryption (Excel 2010+) and Standard Encryption (Excel 2007). Key material is automatically zeroized from memory.

import { writeBlob } from 'modern-xlsx';
const blob = writeBlob(wb);
const url = URL.createObjectURL(blob);
const a = document.createElement('a');
a.href = url;
a.download = 'output.xlsx';
a.click();
URL.revokeObjectURL(url);

Yes! Set the formula property on a cell:

ws.cell('C1').formula = 'SUM(A1:B1)';

The formula will be preserved in the file. Excel will calculate the result when the file is opened.

Use dateToSerial to convert a Date to an Excel serial number, then apply a date number format:

import { dateToSerial } from 'modern-xlsx';
ws.cell('A1').value = dateToSerial(new Date(2024, 0, 15));
const dateStyle = wb.createStyle()
 .numberFormat('yyyy-mm-dd')
 .build(wb.styles);
ws.cell('A1').styleIndex = dateStyle;

Styles in XLSX are shared across the workbook, not stored per-cell. The build(wb.styles) call adds the style definition to the workbook's shared style table and returns an index. Assigning that index to a cell is very efficient — it's just setting a number.

There's no built-in range styling, but you can loop:

const bold = wb.createStyle().font({ bold: true }).build(wb.styles);
for (let col = 0; col < 10; col++) {
 ws.cell(encodeCellRef(0, col)).styleIndex = bold;
}

You forgot to call initWasm() before using the library:

import { initWasm } from 'modern-xlsx';
await initWasm(); // must be called first

Check that you're using the correct number format for dates. Raw serial numbers display as plain numbers unless you apply a date format style.

Writing performance scales with cell count. For very large files (500K+ cells), consider:

• Using aoaToSheet for batch cell creation
• Splitting data across multiple sheets
• Reducing the number of unique styles (each unique combination creates a new style entry)

Use the IIFE bundle from jsDelivr or unpkg:

<script src="https://cdn.jsdelivr.net/npm/modern-xlsx@1.0.0/dist/modern-xlsx.min.js"></script>

The full API is available on window.ModernXlsx. The WASM binary is auto-fetched from the same CDN.

Yes — pass a URL to initWasm():

await ModernXlsx.initWasm('https://my-cdn.com/modern-xlsx.wasm');

Web Workers run XLSX parsing/writing in a separate thread, keeping the main thread responsive for UI interactions. Use createXlsxWorker() to set this up:

const worker = createXlsxWorker({ workerUrl: '/modern-xlsx.worker.js' });
const data = await worker.readBuffer(xlsxBytes);
worker.terminate();

Yes. See the examples in examples/cloudflare-worker/ and examples/deno-deploy/. Both support WASM modules natively.

The IIFE bundle is 29 KB (10 KB gzipped) plus the 939 KB WASM binary. The WASM binary is fetched lazily on initWasm().