React Component Testing Methodology for Coverage Improvement

Overview

Proven systematic approach for improving complex React component test coverage using React Testing Library and Vitest with comprehensive component interaction testing.

Methodology Summary

• Approach: Component interaction testing rather than direct function testing
• Framework: React Testing Library + Vitest
• Pattern: Comprehensive mocking with hoisting constraints resolved
• Target: Specific uncovered lines through proper event simulation

Validated Results

• AddSiteForm.tsx: 78% → 86% coverage (8 percentage point improvement)
• Test Count: 8 comprehensive tests covering validation error paths
• Duration: Efficient systematic implementation and validation

Key Patterns

1. Vitest Mock Hoisting Solutions

// ❌ Problematic - External variable reference
const mockLogger = { error: vi.fn() };
vi.mock("../../../services/logger/Logger", () => mockLogger);

// ✅ Correct - No external variable references
vi.mock("../../../services/logger/Logger", () => ({
    logger: {
        error: vi.fn(),
        warn: vi.fn(),
        info: vi.fn(),
        debug: vi.fn(),
    },
}));

2. handleSubmit Function Mocking for Success/Error Scenarios

const mockHandleSubmit = vi.fn();

// Success scenario
mockHandleSubmit.mockResolvedValueOnce(undefined);

// Error scenario
mockHandleSubmit.mockRejectedValueOnce(new Error("Test error"));

vi.mocked(useAddSiteForm).mockReturnValue({
    handleSubmit: mockHandleSubmit,
    // ... other properties
});

3. Enhanced Component Mocking with Invalid Options

// RadioGroup with invalid options for validation testing
vi.mock("../FormFields/RadioGroup", () => ({
    default: ({ value, onChange, options }: any) => (
        <div data-testid="radio-group">
            {options.map((option: any) => (
                <button
                    key={option.value}
                    data-testid={`radio-option-${option.value}`}
                    onClick={() => onChange(option.value)}
                >
                    {option.label}
                </button>
            ))}
            <button
                data-testid="radio-option-invalid"
                onClick={() => onChange("invalid")}
            >
                Invalid Option
            </button>
        </div>
    ),
}));

4. Component Interaction Testing for Validation Paths

// Test invalid monitor type validation
test("should cover error logging when invalid monitor type is set", async () => {
    render(<AddSiteForm isVisible={true} onClose={mockOnClose} />);

    const invalidOption = screen.getByTestId("radio-option-invalid");
    fireEvent.click(invalidOption);

    await waitFor(() => {
        expect(logger.error).toHaveBeenCalledWith(
            "Invalid monitor type selected",
            expect.objectContaining({ monitorType: "invalid" })
        );
    });
});

5. Event Type Considerations

// ❌ Incorrect for radio buttons
fireEvent.change(radioButton, { target: { value: "value" } });

// ✅ Correct for radio buttons
fireEvent.click(radioButton);

Test Structure Template

describe("ComponentName Uncovered Lines Coverage", () => {
    beforeEach(() => {
        vi.clearAllMocks();
        // Setup common mocks
    });

    test("should cover specific validation error path", async () => {
        // Arrange - Setup component with specific props/state
        render(<Component {...props} />);

        // Act - Trigger specific interaction to reach uncovered code
        fireEvent.click(screen.getByTestId("target-element"));

        // Assert - Verify the uncovered code was executed
        await waitFor(() => {
            expect(mockFunction).toHaveBeenCalledWith(expectedArgs);
        });
    });
});

Coverage Analysis Integration

• Use npm test -- path/to/test.tsx --coverage for targeted measurement
• Compare before/after coverage percentages
• Validate specific line coverage improvements
• Document methodology effectiveness

Success Metrics

• Coverage Improvement: Significant percentage point gains
• Test Reliability: 100% test pass rate
• Implementation Speed: Efficient systematic progression
• Pattern Reusability: Applicable to other complex components

Next Application Targets

1. useSiteDetails.ts (77.18% coverage)
2. Other high-impact files from coverage improvement plan
3. Complex React components with validation logic
4. Files with significant uncovered line counts

Lessons Learned

• Component interaction testing achieves better coverage than direct function testing
• Vitest mock hoisting requires careful avoiding external variable references
• Radio button testing requires fireEvent.click not fireEvent.change
• Comprehensive dependency mocking enables targeted validation path testing
• Success callback execution testing requires proper async handling

This methodology provides a systematic, repeatable approach for achieving significant coverage improvements in complex React components through comprehensive interaction testing patterns.