Added AI skills
This commit is contained in:
Jose Selesan
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---
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id: solid-ocp-abstraction
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title: SOLID - Open/Closed (Abstraction)
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category: solid-principles
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priority: critical
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tags: [SOLID, OCP, open-closed, abstraction]
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related: [solid-ocp-extension, solid-dip-abstractions, pattern-repository]
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---
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# Open/Closed Principle - Abstraction
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Use abstractions (interfaces and abstract classes) to define stable contracts that allow new implementations without modifying existing code.
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## Bad Example
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```typescript
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// Anti-pattern: Concrete dependencies that require modification for new requirements
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class ReportGenerator {
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generateReport(data: SalesData[], format: string): string {
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let report = '';
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// Calculate totals
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const totalSales = data.reduce((sum, d) => sum + d.amount, 0);
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const averageSale = totalSales / data.length;
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if (format === 'html') {
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report = `
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<html>
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<body>
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<h1>Sales Report</h1>
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<p>Total Sales: $${totalSales}</p>
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<p>Average Sale: $${averageSale.toFixed(2)}</p>
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<table>
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${data.map(d => `<tr><td>${d.date}</td><td>$${d.amount}</td></tr>`).join('')}
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</table>
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</body>
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</html>
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`;
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} else if (format === 'pdf') {
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// PDF generation logic mixed in
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const pdf = new PDFDocument();
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pdf.text('Sales Report');
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pdf.text(`Total: $${totalSales}`);
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// ... more PDF logic
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report = pdf.output();
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} else if (format === 'csv') {
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report = 'Date,Amount\n';
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report += data.map(d => `${d.date},${d.amount}`).join('\n');
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} else if (format === 'excel') {
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// Excel generation logic
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// Adding new format requires modifying this class
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}
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return report;
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}
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}
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// Problems:
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// 1. Adding JSON format requires modifying ReportGenerator
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// 2. All format logic is tightly coupled
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// 3. Testing is difficult
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// 4. No way to extend without modification
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```
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## Good Example
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```typescript
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// Correct approach: Abstractions enable extension without modification
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// Stable abstraction for report data
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interface ReportData {
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readonly title: string;
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readonly generatedAt: Date;
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readonly summary: ReportSummary;
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readonly items: ReportItem[];
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}
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interface ReportSummary {
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readonly totalSales: number;
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readonly averageSale: number;
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readonly itemCount: number;
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}
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interface ReportItem {
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readonly date: string;
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readonly amount: number;
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readonly description?: string;
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}
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// Stable abstraction for formatters
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interface ReportFormatter {
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readonly format: string;
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readonly mimeType: string;
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render(data: ReportData): string | Buffer | Promise<string | Buffer>;
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}
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// Stable abstraction for data sources
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interface ReportDataSource<T> {
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fetch(criteria: ReportCriteria): Promise<T[]>;
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transform(rawData: T[]): ReportData;
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}
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// Report generator depends only on abstractions
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class ReportGenerator {
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constructor(
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private formatters: Map<string, ReportFormatter> = new Map()
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) {}
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registerFormatter(formatter: ReportFormatter): void {
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this.formatters.set(formatter.format, formatter);
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}
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async generate<T>(
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dataSource: ReportDataSource<T>,
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criteria: ReportCriteria,
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format: string
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): Promise<GeneratedReport> {
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const formatter = this.formatters.get(format);
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if (!formatter) {
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throw new UnsupportedFormatError(format, this.getSupportedFormats());
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}
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const rawData = await dataSource.fetch(criteria);
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const reportData = dataSource.transform(rawData);
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const content = await Promise.resolve(formatter.render(reportData));
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return {
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content,
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mimeType: formatter.mimeType,
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format,
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generatedAt: new Date()
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};
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}
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getSupportedFormats(): string[] {
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return Array.from(this.formatters.keys());
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}
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}
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// Concrete formatters - each can be added without modifying ReportGenerator
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class HtmlReportFormatter implements ReportFormatter {
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readonly format = 'html';
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readonly mimeType = 'text/html';
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render(data: ReportData): string {
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return `
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<!DOCTYPE html>
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<html>
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<head><title>${data.title}</title></head>
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<body>
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<h1>${data.title}</h1>
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<p>Generated: ${data.generatedAt.toISOString()}</p>
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<section class="summary">
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<p>Total Sales: $${data.summary.totalSales.toFixed(2)}</p>
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<p>Average Sale: $${data.summary.averageSale.toFixed(2)}</p>
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</section>
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<table>
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<thead><tr><th>Date</th><th>Amount</th></tr></thead>
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<tbody>
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${data.items.map(item =>
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`<tr><td>${item.date}</td><td>$${item.amount}</td></tr>`
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).join('')}
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</tbody>
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</table>
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</body>
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</html>
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`;
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}
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}
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class CsvReportFormatter implements ReportFormatter {
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readonly format = 'csv';
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readonly mimeType = 'text/csv';
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render(data: ReportData): string {
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const header = 'Date,Amount,Description';
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const rows = data.items.map(item =>
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`${item.date},${item.amount},"${item.description || ''}"`
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);
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return [header, ...rows].join('\n');
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}
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}
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class JsonReportFormatter implements ReportFormatter {
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readonly format = 'json';
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readonly mimeType = 'application/json';
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render(data: ReportData): string {
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return JSON.stringify(data, null, 2);
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}
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}
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// Adding Excel format - no modification to existing code
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class ExcelReportFormatter implements ReportFormatter {
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readonly format = 'xlsx';
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readonly mimeType = 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet';
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render(data: ReportData): Buffer {
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const workbook = new ExcelJS.Workbook();
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const sheet = workbook.addWorksheet(data.title);
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sheet.addRow(['Date', 'Amount', 'Description']);
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data.items.forEach(item => {
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sheet.addRow([item.date, item.amount, item.description || '']);
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});
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return workbook.xlsx.writeBuffer();
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}
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}
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// Concrete data source - can create new sources without modification
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class SalesReportDataSource implements ReportDataSource<SalesData> {
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constructor(private salesRepository: SalesRepository) {}
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async fetch(criteria: ReportCriteria): Promise<SalesData[]> {
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return this.salesRepository.findByCriteria(criteria);
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}
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transform(rawData: SalesData[]): ReportData {
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const totalSales = rawData.reduce((sum, d) => sum + d.amount, 0);
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return {
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title: 'Sales Report',
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generatedAt: new Date(),
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summary: {
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totalSales,
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averageSale: rawData.length > 0 ? totalSales / rawData.length : 0,
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itemCount: rawData.length
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},
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items: rawData.map(d => ({
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date: d.date,
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amount: d.amount,
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description: d.productName
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}))
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};
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}
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}
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// Usage
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const generator = new ReportGenerator();
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generator.registerFormatter(new HtmlReportFormatter());
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generator.registerFormatter(new CsvReportFormatter());
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generator.registerFormatter(new JsonReportFormatter());
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generator.registerFormatter(new ExcelReportFormatter());
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const salesDataSource = new SalesReportDataSource(salesRepository);
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const report = await generator.generate(salesDataSource, criteria, 'xlsx');
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```
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## Why
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1. **Stable Core**: The `ReportGenerator` class and interfaces form a stable core that rarely changes.
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2. **Independent Development**: Teams can develop new formatters or data sources independently.
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3. **Composition Over Inheritance**: New functionality is added through composition (registering new implementations) rather than inheritance hierarchies.
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4. **Testing**: Each component can be tested in isolation with mock implementations of interfaces.
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5. **Flexibility**: The same generator works with any combination of data sources and formatters.
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6. **Dependency Inversion**: High-level modules (ReportGenerator) don't depend on low-level modules (specific formatters), both depend on abstractions.
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7. **Plugin System**: Natural foundation for a plugin architecture where third parties can add formatters.
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