Best Practices

Overview

Follow these best practices to create high-quality, performant, and maintainable Shopify extensions with Synapse.

Performance Optimization

Bundle Size

Keep extensions under 50KB

Remove unused imports
Avoid large dependencies
Use tree-shaking
Code splitting

Load Time

Load in under 500ms

Lazy load components
Minimize initial render
Cache API responses
Optimize images

Re-renders

Minimize unnecessary renders

Use React.memo
Optimize useEffect deps
Memoize calculations
Avoid inline functions

Function Speed

Functions under 5ms

Use Rust over JavaScript
Minimize loops
Cache results
Limit query surface

Code Quality

TypeScript Best Practices

Strict Types
Type Guards
Interface Definitions

Use strict TypeScript configuration:

// tsconfig.json
{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true
  }
}

Benefits:

Catch errors early
Better IDE support
Self-documenting code
Easier refactoring

Handle undefined values safely:

// ❌ Bad: Assumes value exists
const country = address.country;

// ✅ Good: Type guard
const country = address?.country ?? 'US';

// ✅ Good: Conditional rendering
if (!address) return null;
const country = address.country;

// ✅ Good: Type narrowing
function isProductVariant(
  item: any
): item is ProductVariant {
  return item.__typename === 'ProductVariant';
}

Define clear interfaces:

interface DeliveryEstimate {
  startDate: string;
  endDate: string;
  confidence: 'high' | 'medium' | 'low';
  method: string;
}

interface ExtensionProps {
  address: Address;
  cartTotal: number;
  onEstimateCalculated?: (estimate: DeliveryEstimate) => void;
}

function Extension({ 
  address, 
  cartTotal,
  onEstimateCalculated 
}: ExtensionProps) {
  // Implementation
}

Component Structure

Single Responsibility

Each component does one thing:

// ❌ Bad: Component does too much
function DeliveryEstimate() {
  const address = useShippingAddress();
  const lines = useCartLines();
  const [estimate, setEstimate] = useState(null);
  
  useEffect(() => {
    // Complex calculation
    // API calls
    // Formatting
  }, [address, lines]);
  
  return (
    <View>
      {/* Complex rendering */}
    </View>
  );
}

// ✅ Good: Separated concerns
function DeliveryEstimate() {
  const estimate = useDeliveryEstimate();
  return <DeliveryDisplay estimate={estimate} />;
}

function useDeliveryEstimate() {
  // Custom hook handles logic
}

function DeliveryDisplay({ estimate }) {
  // Component handles rendering
}

Extract Custom Hooks

Reuse logic with custom hooks:

// hooks/useDeliveryEstimate.ts
export function useDeliveryEstimate() {
  const address = useShippingAddress();
  const [estimate, setEstimate] = useState<DeliveryEstimate | null>(null);
  const [loading, setLoading] = useState(false);
  const [error, setError] = useState<string | null>(null);
  
  useEffect(() => {
    if (!address) return;
    
    setLoading(true);
    calculateDeliveryDate(address)
      .then(setEstimate)
      .catch((err) => setError(err.message))
      .finally(() => setLoading(false));
  }, [address]);
  
  return { estimate, loading, error };
}

// Usage
function Extension() {
  const { estimate, loading, error } = useDeliveryEstimate();
  
  if (loading) return <SkeletonText />;
  if (error) return <ErrorBanner message={error} />;
  if (!estimate) return null;
  
  return <DeliveryDisplay estimate={estimate} />;
}

Memoize Expensive Operations

Use useMemo for calculations:

function Extension() {
  const lines = useCartLines();
  
  // ❌ Bad: Recalculates every render
  const total = lines.reduce((sum, line) => 
    sum + parseFloat(line.cost.totalAmount.amount), 0
  );
  
  // ✅ Good: Only recalculates when lines change
  const total = useMemo(() => 
    lines.reduce((sum, line) => 
      sum + parseFloat(line.cost.totalAmount.amount), 0
    ),
    [lines]
  );
  
  return <Total amount={total} />;
}

Function Optimization

Rust vs JavaScript

When to Use Rust
When to Use JavaScript
Performance Tips

Use Rust for:

Production functions
Performance-critical logic
Complex calculations
Large-scale operations

Benefits:

10-100x faster than JS
Compiled to WebAssembly
Type safety
No garbage collection

Example:

// Fast execution, < 1ms typical
fn calculate_discount(lines: &[CartLine]) -> Vec<Discount> {
    lines.iter()
        .filter_map(|line| {
            if line.quantity >= 10 {
                Some(create_discount(line, 20.0))
            } else {
                None
            }
        })
        .collect()
}

Use JavaScript for:

Rapid prototyping
Simple logic
Development/testing
Easy modifications

Benefits:

Easier to modify
Faster iteration
No compilation
Familiar syntax

Example:

// Simple logic, easy to modify
export function run(input) {
  return {
    discounts: input.cart.lines
      .filter(line => line.quantity >= 10)
      .map(line => createDiscount(line, 20))
  };
}

Optimization techniques:

// ✅ Good: Single pass
let discounts: Vec<_> = lines.iter()
    .filter_map(|line| calculate_discount(line))
    .collect();

// ❌ Bad: Multiple passes
let eligible: Vec<_> = lines.iter()
    .filter(|line| is_eligible(line))
    .collect();
let discounts: Vec<_> = eligible.iter()
    .map(|line| calculate_discount(line))
    .collect();

// ✅ Good: Early return
if input.cart.lines.is_empty() {
    return Ok(FunctionRunResult { 
        operations: vec![] 
    });
}

// ✅ Good: Avoid cloning
fn process_line(line: &CartLine) -> Option<Discount> {
    // Use references, avoid clones
}

Error Handling

Graceful Degradation

Handle failures without breaking:

function Extension() {
  const { data, error } = useQuery();
  
  // ❌ Bad: Throws error
  if (error) throw error;
  
  // ✅ Good: Shows fallback
  if (error) {
    return (
      <Banner status="warning">
        Unable to load delivery estimate. 
        Standard shipping applies.
      </Banner>
    );
  }
  
  // ✅ Good: Degrades gracefully
  if (!data) {
    return <SkeletonText />;  // Loading state
  }
  
  return <Content data={data} />;
}

Error Boundaries

Catch component errors:

import { ErrorBoundary } from '@shopify/ui-extensions-react/checkout';

export default reactExtension(
  'purchase.checkout.block.render',
  () => (
    <ErrorBoundary
      onError={(error) => {
        console.error('Extension error:', error);
        // Log to monitoring service
      }}
    >
      <Extension />
    </ErrorBoundary>
  )
);

Function Error Handling

Never panic in functions:

// ❌ Bad: Panics on error
let quantity = line.quantity.parse::<i64>().unwrap();

// ✅ Good: Returns default
let quantity = line.quantity.parse::<i64>().unwrap_or(0);

// ✅ Good: Returns empty on error
fn run(input: Input) -> Result<FunctionRunResult> {
    let operations = match calculate_operations(&input) {
        Ok(ops) => ops,
        Err(e) => {
            eprintln!("Error: {}", e);
            vec![]  // Return empty, don't crash
        }
    };
    
    Ok(FunctionRunResult { operations })
}

User-Friendly Messages

Explain errors clearly:

// ❌ Bad: Technical jargon
<Banner status="critical">
  GraphQL query failed: Field 'deliveryAddress' not found
</Banner>

// ✅ Good: User-friendly
<Banner status="info">
  Unable to calculate delivery estimate. 
  Please enter your shipping address.
</Banner>

// ✅ Good: Actionable
<Banner status="warning">
  Delivery estimate temporarily unavailable.
  <Button onPress={retry}>Try Again</Button>
</Banner>

Testing

Unit Tests
Integration Tests
Function Tests

Test individual functions:

// utils.test.ts
import { describe, it, expect } from 'vitest';
import { calculateDeliveryDate } from './utils';

describe('calculateDeliveryDate', () => {
  it('adds business days correctly', () => {
    const result = calculateDeliveryDate({
      country: 'US',
      province: 'CA'
    });
    
    expect(result.startDate).toBeDefined();
    expect(result.endDate).toBeDefined();
    expect(result.confidence).toBe('high');
  });
  
  it('handles invalid addresses', () => {
    const result = calculateDeliveryDate({
      country: 'XX'
    });
    
    expect(result.error).toBe('Invalid country');
  });
  
  it('accounts for weekends', () => {
    const result = calculateDeliveryDate({
      country: 'US'
    }, new Date('2025-10-31')); // Friday
    
    // Should skip weekend
    expect(result.startDate).not.toBe('2025-11-01');
  });
});

Test extension rendering:

// Extension.test.tsx
import { render, screen } from '@testing-library/react';
import { Extension } from './Extension';

describe('Extension', () => {
  it('renders delivery estimate', () => {
    render(<Extension address={mockAddress} />);
    
    expect(screen.getByText(/estimated delivery/i)).toBeInTheDocument();
  });
  
  it('shows loading state', () => {
    render(<Extension address={null} />);
    
    expect(screen.getByRole('progressbar')).toBeInTheDocument();
  });
  
  it('handles errors gracefully', async () => {
    mockFetch.mockRejectedValueOnce(new Error('API error'));
    
    render(<Extension address={mockAddress} />);
    
    expect(await screen.findByText(/unable to calculate/i))
      .toBeInTheDocument();
  });
});

Test Shopify Functions:

# Create test input
cat > test-input.json << EOF
{
  "cart": {
    "lines": [
      {
        "quantity": 5,
        "merchandise": {
          "id": "gid://shopify/ProductVariant/123"
        }
      }
    ]
  }
}
EOF

# Test function
cargo run < test-input.json > output.json

# Verify output
cat output.json | jq '.discounts | length'
# Should be > 0 for quantity >= 5

Accessibility

Keyboard Navigation

Support keyboard users:

// ✅ Use Button component
<Button onPress={handleClick}>
  Submit
</Button>

// ❌ Don't use div
<div onClick={handleClick}>Submit</div>

Screen Readers

Label everything:

// ✅ Good labels
<TextField
  label="Gift message"
  value={message}
  onChange={setMessage}
/>

<Icon 
  source="delivery" 
  accessibilityLabel="Delivery truck"
/>

Color Contrast

Ensure readability:

Use Shopify UI components (compliant by default)
Avoid custom colors
Test with contrast checker
Minimum 4.5:1 ratio

Focus States

Visible focus indicators:

Shopify components have focus states
Don’t remove with CSS
Test with Tab key
Ensure focus order logical

Security

Input Validation

Validate all user inputs:

function validateGiftMessage(message: string): string | null {
  if (message.length > 200) {
    return "Message too long (max 200 characters)";
  }
  
  if (/<script|javascript:/i.test(message)) {
    return "Invalid characters detected";
  }
  
  if (!message.trim()) {
    return "Message cannot be empty";
  }
  
  return null; // Valid
}

API Security

Secure API calls:

// ✅ Good: HTTPS, authentication
const response = await fetch('https://api.example.com/endpoint', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${sessionToken}`,
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({ data }),
});

// ❌ Bad: HTTP, no auth
fetch('http://api.example.com/endpoint', {
  method: 'POST',
  body: data,
});

Sensitive Data

Never log or expose:

// ❌ Bad: Logs sensitive data
console.log('Customer data:', customer);
console.log('Payment info:', paymentMethod);

// ✅ Good: Logs safely
console.log('Processing order:', { orderId: order.id });
console.log('Payment method type:', paymentMethod.type);

Deployment Best Practices

Test Locally First

npm run dev
# Test in browser
# Verify all features work

Review MCP Validation

Check validation output before deploying

All APIs valid
No deprecated components
TypeScript compiles
No console errors

Monitor Deployment

Watch logs during deployment:

shopify app function logs --watch

Verify deployment succeeds

Test in Dev Store

Add items to cart
Go through checkout
Test all code paths
Verify on mobile

Monitor Performance

Check loading times
Review error rates
Monitor function execution time
Track user feedback

Documentation

Write clear documentation:

# Delivery Estimate Extension

Shows estimated delivery dates in checkout.

## Features

- Calculates delivery based on shipping address
- Displays date range (e.g., "Nov 5-8")
- Handles international shipping
- Shows loading state while calculating

## Configuration

Configure in Shopify admin:
1. Settings → Checkout → Customize
2. Add "Delivery Estimate" block
3. Position after shipping address
4. Save

## Technical Details

- Target: `purchase.checkout.delivery-address.render-after`
- Uses: `useShippingAddress()` hook
- API: No external calls, pure calculation
- Performance: < 100ms load time

## Troubleshooting

**Not showing?**
- Verify added to checkout editor
- Check shipping address is entered
- Review console for errors

Next Steps

Debugging

Learn debugging techniques

Troubleshooting

Common issues and solutions

Extension Standards

Quality guidelines

Validation

MCP validation process

Get Started

Core Concepts

Building Extensions

Building Functions

Development

Deployment

Advanced

Overview

Performance Optimization

Bundle Size

Load Time

Re-renders

Function Speed

Code Quality

TypeScript Best Practices

Component Structure

Function Optimization

Rust vs JavaScript

Error Handling

Testing

Accessibility

Keyboard Navigation

Screen Readers

Color Contrast

Focus States

Security

Deployment Best Practices

Documentation

Next Steps

Debugging

Troubleshooting

Extension Standards

Validation

​Overview

​Performance Optimization

Bundle Size

Load Time

Re-renders

Function Speed

​Code Quality

​TypeScript Best Practices

​Component Structure

​Function Optimization

​Rust vs JavaScript

​Error Handling

​Testing

​Accessibility

Keyboard Navigation

Screen Readers

Color Contrast

Focus States

​Security

​Deployment Best Practices

​Documentation

​Next Steps

Debugging

Troubleshooting

Extension Standards

Validation

Overview

Performance Optimization

Code Quality

TypeScript Best Practices

Component Structure

Function Optimization

Rust vs JavaScript

Error Handling

Testing

Accessibility

Security

Deployment Best Practices

Documentation

Next Steps