ExcelJS

# ExcelJS

Read and write Excel files in Node.js. Full support for styles, formulas, images, and streaming.

## Setup

```bash
# Install ExcelJS for spreadsheet generation and parsing.
npm install exceljs
```

## Creating a Workbook

```typescript
// src/excel/create.ts — Create an Excel workbook with a styled header row and data.
import ExcelJS from "exceljs";

const workbook = new ExcelJS.Workbook();
workbook.creator = "Report System";
workbook.created = new Date();

const sheet = workbook.addWorksheet("Sales Data", {
  properties: { tabColor: { argb: "FF3498DB" } },
});

// Define columns
sheet.columns = [
  { header: "Product", key: "product", width: 25 },
  { header: "Revenue", key: "revenue", width: 15 },
  { header: "Units Sold", key: "units", width: 12 },
  { header: "Growth", key: "growth", width: 12 },
];

// Style header row
sheet.getRow(1).font = { bold: true, color: { argb: "FFFFFFFF" } };
sheet.getRow(1).fill = {
  type: "pattern",
  pattern: "solid",
  fgColor: { argb: "FF3498DB" },
};

// Add data
const data = [
  { product: "Widget Pro", revenue: 45000, units: 1200, growth: 0.12 },
  { product: "Gadget Plus", revenue: 32000, units: 800, growth: 0.08 },
  { product: "Tool Basic", revenue: 18000, units: 2400, growth: -0.03 },
];

data.forEach((row) => sheet.addRow(row));

// Format numbers
sheet.getColumn("revenue").numFmt = "$#,##0";
sheet.getColumn("growth").numFmt = "0.0%";

await workbook.xlsx.writeFile("sales-report.xlsx");
```

## Formulas

```typescript
// src/excel/formulas.ts — Add formulas for totals, averages, and derived values.
import ExcelJS from "exceljs";

const workbook = new ExcelJS.Workbook();
const sheet = workbook.addWorksheet("Financials");

sheet.columns = [
  { header: "Item", key: "item", width: 20 },
  { header: "Q1", key: "q1", width: 12 },
  { header: "Q2", key: "q2", width: 12 },
  { header: "Total", key: "total", width: 12 },
];

sheet.addRow({ item: "Revenue", q1: 100000, q2: 120000 });
sheet.addRow({ item: "Expenses", q1: 80000, q2: 85000 });
sheet.addRow({ item: "Profit" });

// Formula references
sheet.getCell("D2").value = { formula: "B2+C2" } as any;
sheet.getCell("D3").value = { formula: "B3+C3" } as any;
sheet.getCell("B4").value = { formula: "B2-B3" } as any;
sheet.getCell("C4").value = { formula: "C2-C3" } as any;
sheet.getCell("D4").value = { formula: "D2-D3" } as any;

await workbook.xlsx.writeFile("financials.xlsx");
```

## Conditional Formatting

```typescript
// src/excel/conditional.ts — Highlight cells based on value thresholds.
import ExcelJS from "exceljs";

const workbook = new ExcelJS.Workbook();
const sheet = workbook.addWorksheet("KPIs");

sheet.columns = [
  { header: "Metric", key: "metric", width: 20 },
  { header: "Value", key: "value", width: 15 },
];

sheet.addRows([
  { metric: "Uptime", value: 99.9 },
  { metric: "Error Rate", value: 2.3 },
  { metric: "Response Time (ms)", value: 450 },
]);

// Green for values above target, red for below
sheet.addConditionalFormatting({
  ref: "B2:B4",
  rules: [
    {
      type: "cellIs",
      operator: "greaterThan",
      formulae: [95],
      style: { fill: { type: "pattern", pattern: "solid", bgColor: { argb: "FF27AE60" } } },
      priority: 1,
    },
  ],
});

await workbook.xlsx.writeFile("kpis.xlsx");
```

## Reading Excel Files

```typescript
// src/excel/read.ts — Parse an uploaded Excel file and extract data as objects.
import ExcelJS from "exceljs";

export async function parseExcel(filePath: string) {
  const workbook = new ExcelJS.Workbook();
  await workbook.xlsx.readFile(filePath);

  const sheet = workbook.getWorksheet(1)!;
  const headers: string[] = [];
  const rows: Record<string, any>[] = [];

  sheet.eachRow((row, rowNumber) => {
    if (rowNumber === 1) {
      row.eachCell((cell) => headers.push(String(cell.value)));
    } else {
      const obj: Record<string, any> = {};
      row.eachCell((cell, colNumber) => {
        obj[headers[colNumber - 1]] = cell.value;
      });
      rows.push(obj);
    }
  });

  return rows;
}
```

## Streaming Large Files

```typescript
// src/excel/stream.ts — Write large datasets without holding everything in memory.
// Uses ExcelJS streaming writer for millions of rows.
import ExcelJS from "exceljs";
import fs from "fs";

export async function streamLargeExport(data: AsyncIterable<any[]>, outputPath: string) {
  const workbook = new ExcelJS.stream.xlsx.WorkbookWriter({
    stream: fs.createWriteStream(outputPath),
    useStyles: true,
  });

  const sheet = workbook.addWorksheet("Data");
  sheet.columns = [
    { header: "ID", key: "id", width: 10 },
    { header: "Name", key: "name", width: 30 },
    { header: "Value", key: "value", width: 15 },
  ];

  for await (const batch of data) {
    for (const row of batch) {
      sheet.addRow(row).commit();
    }
  }

  sheet.commit();
  await workbook.commit();
}
```