modern-xlsx

High-performance XLSX read/write for JavaScript & TypeScript, powered by Rust + WASM.

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Full cell styling, data validation, conditional formatting, frozen panes, hyperlinks, comments, sheet protection, and more — features that SheetJS locks behind a paid Pro license — all free and open source.

import { initWasm, Workbook } from 'modern-xlsx';

await initWasm();

const wb = new Workbook();
const ws = wb.addSheet('Sheet1');
ws.cell('A1').value = 'Hello';
ws.cell('B1').value = 42;

const bold = wb.createStyle().font({ bold: true }).build(wb.styles);
ws.cell('A1').styleIndex = bold;

await wb.toFile('output.xlsx');

Performance

Node.js 26, single thread, vs SheetJS (xlsx 0.20.3) — indicative, hardware-dependent:

Operation	modern-xlsx	SheetJS CE
Read (100K rows)	494 ms	2,072 ms	4.2x faster
Read (10K rows)	52 ms	177 ms	3.4x faster
Write (10K rows)	180 ms	159 ms	~parity
Output size (100K rows)	~5 MB	~40 MB	~8x smaller

The biggest wins are read speed (3-4x) and file size (~8x smaller); write throughput and CSV/JSON export are roughly at parity.

~25 KB JS (gzip) + ~1.9 MB WASM. Zero runtime dependencies.

Install

npm install modern-xlsx

Full API documentation: packages/modern-xlsx/README.md

Repository Structure

crates/
  modern-xlsx-core/       Rust core — OOXML parsing, XML generation, ZIP I/O
  modern-xlsx-wasm/       WASM bridge — wasm-bindgen exports

packages/
  modern-xlsx/            npm package — TypeScript API, tests, benchmarks

Architecture

  TypeScript API    Workbook / Worksheet / Cell
                              │ JSON
  WASM boundary     wasm-bindgen bridge
                              │
  Rust core         OOXML parser & writer (quick-xml + zip)

Data crosses the WASM boundary as JSON strings for maximum throughput. The Rust core handles ZIP compression, SAX-style XML parsing, shared string table construction, and style resolution.

Development

# Rust tests (441 tests)
cargo test -p modern-xlsx-core

# WASM build
cd crates/modern-xlsx-wasm && wasm-pack build --target web --release \
  --out-dir ../../packages/modern-xlsx/wasm --no-opt

# TypeScript build + tests (1290 tests)
pnpm -C packages/modern-xlsx build
pnpm -C packages/modern-xlsx test

# Lint
cargo clippy -p modern-xlsx-core -- -D warnings
pnpm -C packages/modern-xlsx lint

Toolchain: Rust 1.96+ / beta 1.97 (Edition 2024) / TypeScript 7 / pnpm 11.9 / Biome 2.5

License

MIT