システム開発は「ライブラリを作る」を基準に考える——帳票出力を例に

// reports/salesReport.ts
export async function generateSalesReportPdf(data: SalesReportData) {
  const doc = new PDFDocument();
  doc.fontSize(18).text('売上レポート', { align: 'center' });
  doc.moveDown();

  let y = 100;
  for (const row of data.rows) {
    if (y > 700) {
      doc.addPage();
      y = 50;
    }
    doc.fontSize(10).text(`${row.date}  ${row.customer}  ${row.amount}`, 50, y);
    y += 20;
  }
  return doc;
}

// reports/inventoryReport.ts
export async function generateInventoryReportPdf(data: InventoryReportData) {
  const doc = new PDFDocument();
  doc.fontSize(18).text('在庫レポート', { align: 'center' });
  doc.moveDown();

  let y = 100;
  for (const row of data.rows) {
    if (y > 700) {
      doc.addPage();
      y = 50;
    }
    doc.fontSize(10).text(`${row.sku}  ${row.name}  ${row.qty}`, 50, y);
    y += 20;
  }
  return doc;
}

// lib/reportBuilder.ts
export class ReportBuilder {
  private doc = new PDFDocument();
  private y = 100;

  constructor(
    title: string,
    private pageBottom = 700
  ) {
    this.doc.fontSize(18).text(title, { align: 'center' });
    this.doc.moveDown();
  }

  addRow(cells: string[], columnWidths: number[]) {
    if (this.y > this.pageBottom) {
      this.doc.addPage();
      this.y = 50;
    }
    let x = 50;
    cells.forEach((cell, i) => {
      this.doc.fontSize(10).text(cell, x, this.y);
      x += columnWidths[i];
    });
    this.y += 20;
  }

  build(): PDFDocument {
    return this.doc;
  }
}

// reports/salesReport.ts
export async function generateSalesReportPdf(data: SalesReportData) {
  const report = new ReportBuilder('売上レポート');
  for (const row of data.rows) {
    report.addRow([row.date, row.customer, String(row.amount)], [80, 150, 80]);
  }
  return report.build();
}