ExcelJS — Read, Write, and Stream Excel Spreadsheets in Node.js

Introduction

ExcelJS provides a complete API for generating and parsing Excel files in Node.js without requiring Microsoft Office. It supports rich formatting, formulas, images, charts metadata, and streaming — making it suitable for server-side report generation and ETL pipelines that handle millions of rows.

What ExcelJS Does

• Creates XLSX workbooks with multiple sheets, cell styles, merged cells, and conditional formatting
• Reads existing XLSX files preserving formulas, data validations, and defined names
• Streams rows to disk or HTTP responses for memory-efficient export of large datasets
• Supports CSV reading and writing with configurable delimiters and encoding
• Handles rich cell types including dates, hyperlinks, rich text, and images

Architecture Overview

ExcelJS models workbooks as in-memory object graphs of worksheets, rows, and cells. The XLSX serializer uses a streaming XML writer (SAX-style) that flushes sheet data in chunks, preventing memory exhaustion on large exports. For reading, it provides both a full-parse mode (entire workbook in memory) and a streaming reader that emits row events, suitable for processing files that exceed available RAM.

Self-Hosting & Configuration

• Install from npm — pure JavaScript, no native bindings required
• Use workbook.xlsx.writeFile(path) for disk output or workbook.xlsx.write(stream) for HTTP responses
• Enable streaming writes via new ExcelJS.stream.xlsx.WorkbookWriter({ stream }) for constant-memory exports
• Configure default column widths, row heights, and page setup (orientation, margins) per worksheet
• Set number formats, fonts, borders, and fills at the cell or column level

Key Features

• Streaming writer keeps memory flat regardless of row count — tested with 1M+ rows
• Full style support: fonts, fills, borders, alignment, number formats, and conditional formatting
• Data validation rules (dropdowns, numeric ranges) embedded directly in cells
• Auto-filter and freeze-pane support for user-friendly spreadsheets
• Over 15,000 GitHub stars and 4 million weekly npm downloads

Comparison with Similar Tools

• SheetJS (xlsx) — broader format support (XLS, ODS); ExcelJS offers richer styling and streaming writes
• openpyxl (Python) — Python equivalent; ExcelJS is the Node.js counterpart with similar capabilities
• Apache POI (Java) — enterprise-grade but JVM-only; ExcelJS runs anywhere Node.js does
• csv-parser — CSV-only streaming reader; ExcelJS handles both XLSX and CSV in one library
• json2csv — converts JSON to CSV; ExcelJS produces formatted Excel files with charts and styles

FAQ

Q: Can ExcelJS handle files with 100K+ rows? A: Yes. Use the streaming WorkbookWriter for writing and the streaming reader for parsing to maintain constant memory usage.

Q: Does it support Excel formulas? A: Yes. You can set formula strings on cells. Results are calculated by Excel when the file is opened, not by ExcelJS itself.

Q: Can I add images to spreadsheets? A: Yes. Use worksheet.addImage() with a buffer or file path. Supported formats include PNG, JPEG, and GIF.

Q: Is it compatible with LibreOffice and Google Sheets? A: The generated XLSX files conform to the Open XML standard and open correctly in LibreOffice Calc and Google Sheets.

Sources

• https://github.com/exceljs/exceljs
• https://github.com/exceljs/exceljs/blob/master/README.md