GitHub Copilot Mastery Part 4: Advanced Usage Techniques

// PATTERN: Repository with caching, error handling, and metrics
// Each method should: validate input, check cache, query DB, update cache, emit metrics

interface UserRepository {
    // Pattern established - Copilot will follow for all methods
    async findById(id: string): Promise<User | null> {
        // Validate input
        if (!id || typeof id !== 'string') {
            this.metrics.increment('user.repository.invalid_input');
            throw new ValidationError('Invalid user ID');
        }

        // Check cache first
        const cached = await this.cache.get(`user:${id}`);
        if (cached) {
            this.metrics.increment('user.repository.cache_hit');
            return JSON.parse(cached);
        }

        // Query database
        try {
            const user = await this.db.query('SELECT * FROM users WHERE id = ?', [id]);
            if (user) {
                await this.cache.set(`user:${id}`, JSON.stringify(user), 300);
                this.metrics.increment('user.repository.cache_miss');
                return user;
            }
            return null;
        } catch (error) {
            this.metrics.increment('user.repository.error');
            throw new DatabaseError('Failed to fetch user', error);
        }
    }

    // Copilot will now follow the established pattern for other methods
    async findByEmail(email: string): Promise<User | null> {
        // Similar pattern implementation suggested
    }
}

// examples/api-patterns.ts
/**
 * Standard API endpoint pattern
 * Use this template for all new endpoints
 */
export const standardEndpoint = {
    // Input validation with Zod
    validator: z.object({
        id: z.string().uuid(),
        name: z.string().min(1).max(100),
        email: z.string().email()
    }),

    // Handler with proper error handling
    handler: async (req: Request, res: Response) => {
        const correlationId = generateCorrelationId();
        
        try {
            // Validate input
            const input = validator.parse(req.body);
            
            // Business logic
            const result = await businessLogic.process(input);
            
            // Success response
            return res.status(200).json({
                success: true,
                data: result,
                correlationId
            });
            
        } catch (error) {
            logger.error('Endpoint error', { error, correlationId });
            
            if (error instanceof ValidationError) {
                return res.status(400).json({
                    success: false,
                    error: 'Invalid input',
                    correlationId
                });
            }
            
            return res.status(500).json({
                success: false,
                error: 'Internal server error',
                correlationId
            });
        }
    }
};

// types/domain.ts
/**
 * Core domain types for the application
 * These interfaces should be referenced in all related code
 */

export interface User {
    id: string;
    email: string;
    name: string;
    role: UserRole;
    permissions: Permission[];
    createdAt: Date;
    updatedAt: Date;
    lastLoginAt?: Date;
    isActive: boolean;
    metadata: UserMetadata;
}

export interface UserMetadata {
    preferences: UserPreferences;
    settings: UserSettings;
    profileCompletion: number; // 0-100
    verificationStatus: VerificationStatus;
}

// Include comprehensive JSDoc comments
/**
 * Represents a complete e-commerce order
 * @example
 * const order: Order = {
 *   id: "ord_123",
 *   customerId: "usr_456", 
 *   status: OrderStatus.PENDING,
 *   items: [{ productId: "prod_789", quantity: 2, price: 29.99 }],
 *   total: 59.98,
 *   createdAt: new Date()
 * };
 */
export interface Order {
    id: string;
    customerId: string;
    status: OrderStatus;
    items: OrderItem[];
    total: number;
    currency: string;
    shippingAddress: Address;
    billingAddress: Address;
    paymentMethod: PaymentMethod;
    createdAt: Date;
    updatedAt: Date;
    fulfillmentDate?: Date;
    trackingNumber?: string;
}

// mcp-server/company-standards.ts
import { MCPServer } from '@mcp/sdk';

class CompanyStandardsMCP extends MCPServer {
    async getCodeStandards(language: string) {
        return {
            linting: await this.loadLintingRules(language),
            testing: await this.loadTestingPatterns(language),
            documentation: await this.loadDocPatterns(language),
            security: await this.loadSecurityGuidelines(language)
        };
    }
    
    async validateArchitecture(codebase: string) {
        return {
            violations: await this.checkArchitectureViolations(codebase),
            suggestions: await this.generateImprovements(codebase),
            compliance: await this.checkComplianceRules(codebase)
        };
    }
}

// Custom tool that uses Copilot API
class CustomCodeGenerator {
    async generateWithContext(prompt: string, context: ProjectContext) {
        const enhancedPrompt = this.buildContextualPrompt(prompt, context);
        
        const response = await copilotAPI.complete({
            prompt: enhancedPrompt,
            maxTokens: 1000,
            temperature: 0.7,
            context: {
                files: context.relevantFiles,
                dependencies: context.projectDependencies,
                patterns: context.codingPatterns
            }
        });
        
        return this.postProcessResponse(response);
    }
}

// Organize imports to provide clear context
import { Database } from './core/database';
import { Logger } from './utils/logger';
import { ValidationError, DatabaseError } from './errors';
import { UserRepository } from './repositories/user';
import type { User, CreateUserInput } from './types/user';

// Clear, contextual class definition
export class UserService {
    // Properties provide context for method suggestions
    private db: Database;
    private logger: Logger;
    private userRepo: UserRepository;
    
    // Method signatures help Copilot understand patterns
    async createUser(input: CreateUserInput): Promise<Result<User, ValidationError>> {
        // Implementation with clear error handling patterns
    }
}

// Custom metrics for Copilot effectiveness
class CopilotMetrics {
    track(suggestion: CopilotSuggestion, outcome: 'accepted' | 'rejected' | 'modified') {
        this.metrics.increment(`copilot.suggestion.${outcome}`);
        
        if (outcome === 'modified') {
            this.analyzeModifications(suggestion.original, suggestion.modified);
        }
    }
    
    generateEffectivenessReport() {
        return {
            acceptanceRate: this.calculateAcceptanceRate(),
            topPatterns: this.findMostAcceptedPatterns(),
            improvements: this.suggestImprovements()
        };
    }
}