Core Components

Design System Overview

Use components from your core library instead of raw platform components. This ensures consistent styling and behavior.

Design Tokens

NEVER hard-code values. Always use design tokens.

Spacing Tokens

// CORRECT - Use tokens
<Box padding="$4" marginBottom="$2" />

// WRONG - Hard-coded values
<Box padding={16} marginBottom={8} />

Color Tokens

// CORRECT - Semantic tokens
<Text color="$textPrimary" />
<Box backgroundColor="$backgroundSecondary" />

// WRONG - Hard-coded colors
<Text color="#333333" />
<Box backgroundColor="rgb(245, 245, 245)" />

Typography Tokens

<Text fontSize="$lg" fontWeight="$semibold" />

Core Components

Box

Base layout component with token support:

<Box
  padding="$4"
  backgroundColor="$backgroundPrimary"
  borderRadius="$lg"
>
  {children}
</Box>

HStack / VStack

Horizontal and vertical flex layouts:

<HStack gap="$3" alignItems="center">
  <Icon name="user" />
  <Text>Username</Text>
</HStack>

<VStack gap="$4" padding="$4">
  <Heading>Title</Heading>
  <Text>Content</Text>
</VStack>

Text

Typography with token support:

<Text
  fontSize="$lg"
  fontWeight="$semibold"
  color="$textPrimary"
>
  Hello World
</Text>

Button

Interactive button with variants:

<Button
  onPress={handlePress}
  variant="solid"
  size="md"
  isLoading={loading}
  isDisabled={disabled}
>
  Click Me
</Button>

Input

Form input with validation:

<Input
  value={value}
  onChangeText={setValue}
  placeholder="Enter text"
  error={touched ? errors.field : undefined}
  label="Field Name"
/>

Card

Content container:

<Card padding="$4" gap="$3">
  <CardHeader>
    <Heading size="sm">Card Title</Heading>
  </CardHeader>
  <CardBody>
    <Text>Card content</Text>
  </CardBody>
</Card>

Layout Patterns

Screen Layout

const MyScreen = () => (
  <Screen>
    <ScreenHeader title="Page Title" />
    <ScreenContent padding="$4">
      {/* Content */}
    </ScreenContent>
  </Screen>
);

Form Layout

<VStack gap="$4" padding="$4">
  <Input label="Name" {...nameProps} />
  <Input label="Email" {...emailProps} />
  <Button isLoading={loading}>Submit</Button>
</VStack>

List Item Layout

<HStack
  padding="$4"
  gap="$3"
  alignItems="center"
  borderBottomWidth={1}
  borderColor="$borderLight"
>
  <Avatar source={{ uri: imageUrl }} size="md" />
  <VStack flex={1}>
    <Text fontWeight="$semibold">{title}</Text>
    <Text color="$textSecondary" fontSize="$sm">{subtitle}</Text>
  </VStack>
  <Icon name="chevron-right" color="$textTertiary" />
</HStack>

Anti-Patterns

// WRONG - Hard-coded values
<View style={{ padding: 16, backgroundColor: '#fff' }}>

// CORRECT - Design tokens
<Box padding="$4" backgroundColor="$backgroundPrimary">


// WRONG - Raw platform components
import { View, Text } from 'react-native';

// CORRECT - Core components
import { Box, Text } from 'components/core';


// WRONG - Inline styles
<Text style={{ fontSize: 18, fontWeight: '600' }}>

// CORRECT - Token props
<Text fontSize="$lg" fontWeight="$semibold">

Component Props Pattern

When creating components, use token-based props:

interface CardProps {
  padding?: '$2' | '$4' | '$6';
  variant?: 'elevated' | 'outlined' | 'filled';
  children: React.ReactNode;
}

const Card = ({ padding = '$4', variant = 'elevated', children }: CardProps) => (
  <Box
    padding={padding}
    backgroundColor="$backgroundPrimary"
    borderRadius="$lg"
    {...variantStyles[variant]}
  >
    {children}
  </Box>
);

Integration with Other Skills

• react-ui-patterns: Use core components for UI states
• testing-patterns: Mock core components in tests
• storybook: Document component variants

Formik Patterns

Basic Form Setup

import { useFormik } from 'formik';
import * as yup from 'yup';

const validationSchema = yup.object({
  email: yup.string().email('Invalid email').required('Email is required'),
  password: yup.string().min(8, 'Min 8 characters').required('Password is required'),
});

const LoginForm = () => {
  const formik = useFormik({
    initialValues: {
      email: '',
      password: '',
    },
    validationSchema,
    onSubmit: async (values) => {
      await loginMutation({ variables: { input: values } });
    },
  });

  return (
    <VStack gap="$4">
      <Input
        label="Email"
        value={formik.values.email}
        onChangeText={formik.handleChange('email')}
        onBlur={formik.handleBlur('email')}
        error={formik.touched.email ? formik.errors.email : undefined}
        keyboardType="email-address"
        autoCapitalize="none"
      />

      <Input
        label="Password"
        value={formik.values.password}
        onChangeText={formik.handleChange('password')}
        onBlur={formik.handleBlur('password')}
        error={formik.touched.password ? formik.errors.password : undefined}
        secureTextEntry
      />

      <Button
        onPress={formik.handleSubmit}
        isDisabled={!formik.isValid || formik.isSubmitting}
        isLoading={formik.isSubmitting}
      >
        Login
      </Button>
    </VStack>
  );
};

Validation Schemas

Common Patterns

import * as yup from 'yup';

// Email
email: yup.string()
  .email('Invalid email address')
  .required('Email is required')

// Password with requirements
password: yup.string()
  .min(8, 'Must be at least 8 characters')
  .matches(/[a-z]/, 'Must contain lowercase letter')
  .matches(/[A-Z]/, 'Must contain uppercase letter')
  .matches(/[0-9]/, 'Must contain number')
  .required('Password is required')

// Confirm password
confirmPassword: yup.string()
  .oneOf([yup.ref('password')], 'Passwords must match')
  .required('Please confirm password')

// Phone number
phone: yup.string()
  .matches(/^\+?[1-9]\d{1,14}$/, 'Invalid phone number')
  .required('Phone is required')

// Optional field with validation when present
website: yup.string()
  .url('Must be a valid URL')
  .nullable()

// Number with range
quantity: yup.number()
  .min(1, 'Minimum 1')
  .max(100, 'Maximum 100')
  .required('Quantity required')

// Array with minimum items
tags: yup.array()
  .of(yup.string())
  .min(1, 'Select at least one tag')

Conditional Validation

const schema = yup.object({
  hasCompany: yup.boolean(),
  companyName: yup.string().when('hasCompany', {
    is: true,
    then: (schema) => schema.required('Company name required'),
    otherwise: (schema) => schema.nullable(),
  }),
});

Form Field Helpers

Input Helper

const getFieldProps = (name: keyof typeof formik.values) => ({
  value: formik.values[name],
  onChangeText: formik.handleChange(name),
  onBlur: formik.handleBlur(name),
  error: formik.touched[name] ? formik.errors[name] : undefined,
});

// Usage
<Input label="Email" {...getFieldProps('email')} />

Select/Picker Helper

<Select
  label="Country"
  value={formik.values.country}
  onValueChange={(value) => formik.setFieldValue('country', value)}
  error={formik.touched.country ? formik.errors.country : undefined}
  options={countryOptions}
/>

Form Submission with GraphQL

const CreateItemForm = () => {
  const [createItem] = useCreateItemMutation({
    onCompleted: () => {
      toast.success({ title: 'Item created' });
      navigation.goBack();
    },
    onError: (error) => {
      console.error('createItem failed:', error);
      toast.error({ title: 'Failed to create item' });
    },
  });

  const formik = useFormik({
    initialValues: { name: '', description: '' },
    validationSchema,
    onSubmit: async (values, { setSubmitting }) => {
      try {
        await createItem({ variables: { input: values } });
      } finally {
        setSubmitting(false);
      }
    },
  });

  return (
    <VStack gap="$4">
      {/* Form fields */}
      <Button
        onPress={formik.handleSubmit}
        isDisabled={!formik.isValid || formik.isSubmitting}
        isLoading={formik.isSubmitting}
      >
        Create
      </Button>
    </VStack>
  );
};

Edit Form with Initial Values

const EditItemForm = ({ item }: { item: Item }) => {
  const [updateItem] = useUpdateItemMutation({
    onCompleted: () => toast.success({ title: 'Saved' }),
    onError: (error) => {
      console.error('updateItem failed:', error);
      toast.error({ title: 'Save failed' });
    },
  });

  const formik = useFormik({
    initialValues: {
      name: item.name,
      description: item.description ?? '',
    },
    enableReinitialize: true, // Update when item prop changes
    validationSchema,
    onSubmit: async (values) => {
      await updateItem({
        variables: { id: item.id, input: values },
      });
    },
  });

  // Track if form has changes
  const hasChanges = formik.dirty;

  return (
    <VStack gap="$4">
      {/* Form fields */}
      <Button
        onPress={formik.handleSubmit}
        isDisabled={!hasChanges || !formik.isValid || formik.isSubmitting}
        isLoading={formik.isSubmitting}
      >
        Save Changes
      </Button>
    </VStack>
  );
};

Form State Helpers

const {
  values,          // Current form values
  errors,          // Validation errors
  touched,         // Fields that have been touched
  isValid,         // Form passes validation
  isSubmitting,    // Submit in progress
  dirty,           // Values differ from initial
  handleSubmit,    // Submit handler
  handleChange,    // Change handler
  handleBlur,      // Blur handler
  setFieldValue,   // Set single field
  setFieldTouched, // Mark field touched
  resetForm,       // Reset to initial values
  setSubmitting,   // Control submitting state
} = formik;

Multi-Step Forms

const MultiStepForm = () => {
  const [step, setStep] = useState(0);

  const formik = useFormik({
    initialValues: {
      // Step 1
      name: '',
      email: '',
      // Step 2
      address: '',
      city: '',
      // Step 3
      cardNumber: '',
    },
    validationSchema: stepSchemas[step],
    onSubmit: async (values) => {
      if (step < steps.length - 1) {
        setStep(step + 1);
      } else {
        await submitOrder(values);
      }
    },
  });

  return (
    <VStack>
      {step === 0 && <PersonalInfoStep formik={formik} />}
      {step === 1 && <AddressStep formik={formik} />}
      {step === 2 && <PaymentStep formik={formik} />}

      <HStack gap="$4">
        {step > 0 && (
          <Button variant="outline" onPress={() => setStep(step - 1)}>
            Back
          </Button>
        )}
        <Button
          onPress={formik.handleSubmit}
          isDisabled={!formik.isValid}
          isLoading={formik.isSubmitting}
        >
          {step < steps.length - 1 ? 'Next' : 'Submit'}
        </Button>
      </HStack>
    </VStack>
  );
};

Anti-Patterns

// WRONG - Not showing validation errors
<Input
  value={formik.values.email}
  onChangeText={formik.handleChange('email')}
/>

// CORRECT - Show errors when touched
<Input
  value={formik.values.email}
  onChangeText={formik.handleChange('email')}
  onBlur={formik.handleBlur('email')}
  error={formik.touched.email ? formik.errors.email : undefined}
/>


// WRONG - Submit button always enabled
<Button onPress={formik.handleSubmit}>Submit</Button>

// CORRECT - Disabled when invalid or submitting
<Button
  onPress={formik.handleSubmit}
  isDisabled={!formik.isValid || formik.isSubmitting}
  isLoading={formik.isSubmitting}
>
  Submit
</Button>


// WRONG - No error handling on mutation
onSubmit: async (values) => {
  await createItem({ variables: { input: values } });
}

// CORRECT - Handle errors
onSubmit: async (values, { setSubmitting }) => {
  try {
    await createItem({ variables: { input: values } });
  } catch (error) {
    toast.error({ title: 'Failed to save' });
  } finally {
    setSubmitting(false);
  }
}

Integration with Other Skills

• graphql-schema: Mutation submission patterns
• react-ui-patterns: Loading/error states
• testing-patterns: Test form validation and submission

GraphQL Schema Patterns

Core Rules

1. NEVER inline gql literals - Create .gql files
2. ALWAYS run codegen after creating/modifying .gql files
3. ALWAYS add onError handler to mutations
4. Use generated hooks - Never write raw Apollo hooks

File Structure

src/
├── components/
│   └── ItemList/
│       ├── ItemList.tsx
│       ├── GetItems.gql           # Query definition
│       └── GetItems.generated.ts  # Auto-generated (don't edit)
└── graphql/
    └── mutations/
        └── CreateItem.gql         # Shared mutations

Creating a Query

Step 1: Create .gql file

# src/components/ItemList/GetItems.gql
query GetItems($limit: Int, $offset: Int) {
  items(limit: $limit, offset: $offset) {
    id
    name
    description
    createdAt
  }
}

Step 2: Run codegen

npm run gql:typegen

Step 3: Import and use generated hook

import { useGetItemsQuery } from './GetItems.generated';

const ItemList = () => {
  const { data, loading, error, refetch } = useGetItemsQuery({
    variables: { limit: 20, offset: 0 },
  });

  if (error) return <ErrorState error={error} onRetry={refetch} />;
  if (loading && !data) return <LoadingSkeleton />;
  if (!data?.items.length) return <EmptyState />;

  return <List items={data.items} />;
};

Creating a Mutation

Step 1: Create .gql file

# src/graphql/mutations/CreateItem.gql
mutation CreateItem($input: CreateItemInput!) {
  createItem(input: $input) {
    id
    name
    description
  }
}

Step 2: Run codegen

npm run gql:typegen

Step 3: Use with REQUIRED error handling

import { useCreateItemMutation } from 'graphql/mutations/CreateItem.generated';

const CreateItemForm = () => {
  const [createItem, { loading }] = useCreateItemMutation({
    // Success handling
    onCompleted: (data) => {
      toast.success({ title: 'Item created' });
      navigation.goBack();
    },
    // ERROR HANDLING IS REQUIRED
    onError: (error) => {
      console.error('createItem failed:', error);
      toast.error({ title: 'Failed to create item' });
    },
    // Cache update
    update: (cache, { data }) => {
      if (data?.createItem) {
        cache.modify({
          fields: {
            items: (existing = []) => [...existing, data.createItem],
          },
        });
      }
    },
  });

  return (
    <Button
      onPress={() => createItem({ variables: { input: formValues } })}
      isDisabled={!isValid || loading}
      isLoading={loading}
    >
      Create
    </Button>
  );
};

Mutation UI Requirements

CRITICAL: Every mutation trigger must:

1. Be disabled during mutation - Prevent double-clicks
2. Show loading state - Visual feedback
3. Have onError handler - User knows it failed
4. Show success feedback - User knows it worked

// CORRECT - Complete mutation pattern
const [submit, { loading }] = useSubmitMutation({
  onError: (error) => {
    console.error('submit failed:', error);
    toast.error({ title: 'Save failed' });
  },
  onCompleted: () => {
    toast.success({ title: 'Saved' });
  },
});

<Button
  onPress={handleSubmit}
  isDisabled={!isValid || loading}
  isLoading={loading}
>
  Submit
</Button>

Query Options

Fetch Policies

Common Options

useGetItemsQuery({
  variables: { id: itemId },

  // Fetch strategy
  fetchPolicy: 'cache-and-network',

  // Re-render on network status changes
  notifyOnNetworkStatusChange: true,

  // Skip if condition not met
  skip: !itemId,

  // Poll for updates
  pollInterval: 30000,
});

Optimistic Updates

For instant UI feedback:

const [toggleFavorite] = useToggleFavoriteMutation({
  optimisticResponse: {
    toggleFavorite: {
      __typename: 'Item',
      id: itemId,
      isFavorite: !currentState,
    },
  },
  onError: (error) => {
    // Rollback happens automatically
    console.error('toggleFavorite failed:', error);
    toast.error({ title: 'Failed to update' });
  },
});

When NOT to Use Optimistic Updates

• Operations that can fail validation
• Operations with server-generated values
• Destructive operations (delete)
• Operations affecting other users

Fragments

For reusable field selections:

# src/graphql/fragments/ItemFields.gql
fragment ItemFields on Item {
  id
  name
  description
  createdAt
  updatedAt
}

Use in queries:

query GetItems {
  items {
    ...ItemFields
  }
}

Anti-Patterns

// WRONG - Inline gql
const GET_ITEMS = gql`
  query GetItems { items { id } }
`;

// CORRECT - Use .gql file + generated hook
import { useGetItemsQuery } from './GetItems.generated';


// WRONG - No error handler
const [mutate] = useMutation(MUTATION);

// CORRECT - Always handle errors
const [mutate] = useMutation(MUTATION, {
  onError: (error) => {
    console.error('mutation failed:', error);
    toast.error({ title: 'Operation failed' });
  },
});


// WRONG - Button not disabled during mutation
<Button onPress={submit}>Submit</Button>

// CORRECT - Disabled and loading
<Button onPress={submit} isDisabled={loading} isLoading={loading}>
  Submit
</Button>

Codegen Commands

# Generate types from .gql files
npm run gql:typegen

# Download schema + generate types
npm run sync-types

Integration with Other Skills

• react-ui-patterns: Loading/error/empty states for queries
• testing-patterns: Mock generated hooks in tests
• formik-patterns: Mutation submission patterns

React UI Patterns

Core Principles

1. Never show stale UI - Loading spinners only when actually loading
2. Always surface errors - Users must know when something fails
3. Optimistic updates - Make the UI feel instant
4. Progressive disclosure - Show content as it becomes available
5. Graceful degradation - Partial data is better than no data

Loading State Patterns

The Golden Rule

Show loading indicator ONLY when there's no data to display.

// CORRECT - Only show loading when no data exists
const { data, loading, error } = useGetItemsQuery();

if (error) return <ErrorState error={error} onRetry={refetch} />;
if (loading && !data) return <LoadingState />;
if (!data?.items.length) return <EmptyState />;

return <ItemList items={data.items} />;

// WRONG - Shows spinner even when we have cached data
if (loading) return <LoadingState />; // Flashes on refetch!

Loading State Decision Tree

Is there an error?
  → Yes: Show error state with retry option
  → No: Continue

Is it loading AND we have no data?
  → Yes: Show loading indicator (spinner/skeleton)
  → No: Continue

Do we have data?
  → Yes, with items: Show the data
  → Yes, but empty: Show empty state
  → No: Show loading (fallback)

Skeleton vs Spinner

Error Handling Patterns

The Error Handling Hierarchy

1. Inline error (field-level) → Form validation errors
2. Toast notification → Recoverable errors, user can retry
3. Error banner → Page-level errors, data still partially usable
4. Full error screen → Unrecoverable, needs user action

Always Show Errors

CRITICAL: Never swallow errors silently.

// CORRECT - Error always surfaced to user
const [createItem, { loading }] = useCreateItemMutation({
  onCompleted: () => {
    toast.success({ title: 'Item created' });
  },
  onError: (error) => {
    console.error('createItem failed:', error);
    toast.error({ title: 'Failed to create item' });
  },
});

// WRONG - Error silently caught, user has no idea
const [createItem] = useCreateItemMutation({
  onError: (error) => {
    console.error(error); // User sees nothing!
  },
});

Error State Component Pattern

interface ErrorStateProps {
  error: Error;
  onRetry?: () => void;
  title?: string;
}

const ErrorState = ({ error, onRetry, title }: ErrorStateProps) => (
  <div className="error-state">
    <Icon name="exclamation-circle" />
    <h3>{title ?? 'Something went wrong'}</h3>
    <p>{error.message}</p>
    {onRetry && (
      <Button onClick={onRetry}>Try Again</Button>
    )}
  </div>
);

Button State Patterns

Button Loading State

<Button
  onClick={handleSubmit}
  isLoading={isSubmitting}
  disabled={!isValid || isSubmitting}
>
  Submit
</Button>

Disable During Operations

CRITICAL: Always disable triggers during async operations.

// CORRECT - Button disabled while loading
<Button
  disabled={isSubmitting}
  isLoading={isSubmitting}
  onClick={handleSubmit}
>
  Submit
</Button>

// WRONG - User can tap multiple times
<Button onClick={handleSubmit}>
  {isSubmitting ? 'Submitting...' : 'Submit'}
</Button>

Empty States

Empty State Requirements

Every list/collection MUST have an empty state:

// WRONG - No empty state
return <FlatList data={items} />;

// CORRECT - Explicit empty state
return (
  <FlatList
    data={items}
    ListEmptyComponent={<EmptyState />}
  />
);

Contextual Empty States

// Search with no results
<EmptyState
  icon="search"
  title="No results found"
  description="Try different search terms"
/>

// List with no items yet
<EmptyState
  icon="plus-circle"
  title="No items yet"
  description="Create your first item"
  action={{ label: 'Create Item', onClick: handleCreate }}
/>

Form Submission Pattern

const MyForm = () => {
  const [submit, { loading }] = useSubmitMutation({
    onCompleted: handleSuccess,
    onError: handleError,
  });

  const handleSubmit = async () => {
    if (!isValid) {
      toast.error({ title: 'Please fix errors' });
      return;
    }
    await submit({ variables: { input: values } });
  };

  return (
    <form>
      <Input
        value={values.name}
        onChange={handleChange('name')}
        error={touched.name ? errors.name : undefined}
      />
      <Button
        type="submit"
        onClick={handleSubmit}
        disabled={!isValid || loading}
        isLoading={loading}
      >
        Submit
      </Button>
    </form>
  );
};

Anti-Patterns

Loading States

// WRONG - Spinner when data exists (causes flash)
if (loading) return <Spinner />;

// CORRECT - Only show loading without data
if (loading && !data) return <Spinner />;

Error Handling

// WRONG - Error swallowed
try {
  await mutation();
} catch (e) {
  console.log(e); // User has no idea!
}

// CORRECT - Error surfaced
onError: (error) => {
  console.error('operation failed:', error);
  toast.error({ title: 'Operation failed' });
}

Button States

// WRONG - Button not disabled during submission
<Button onClick={submit}>Submit</Button>

// CORRECT - Disabled and shows loading
<Button onClick={submit} disabled={loading} isLoading={loading}>
  Submit
</Button>

Checklist

Before completing any UI component:

UI States:

• Error state handled and shown to user
• Loading state shown only when no data exists
• Empty state provided for collections
• Buttons disabled during async operations
• Buttons show loading indicator when appropriate

Data & Mutations:

• Mutations have onError handler
• All user actions have feedback (toast/visual)

Integration with Other Skills

• graphql-schema: Use mutation patterns with proper error handling
• testing-patterns: Test all UI states (loading, error, empty, success)
• formik-patterns: Apply form submission patterns

Systematic Debugging

Core Principle

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST.

Never apply symptom-focused patches that mask underlying problems. Understand WHY something fails before attempting to fix it.

The Four-Phase Framework

Phase 1: Root Cause Investigation

Before touching any code:

1. Read error messages thoroughly - Every word matters
2. Reproduce the issue consistently - If you can't reproduce it, you can't verify a fix
3. Examine recent changes - What changed before this started failing?
4. Gather diagnostic evidence - Logs, stack traces, state dumps
5. Trace data flow - Follow the call chain to find where bad values originate

Root Cause Tracing Technique:

1. Observe the symptom - Where does the error manifest?
2. Find immediate cause - Which code directly produces the error?
3. Ask "What called this?" - Map the call chain upward
4. Keep tracing up - Follow invalid data backward through the stack
5. Find original trigger - Where did the problem actually start?

Key principle: Never fix problems solely where errors appear—always trace to the original trigger.

Phase 2: Pattern Analysis

1. Locate working examples - Find similar code that works correctly
2. Compare implementations completely - Don't just skim
3. Identify differences - What's different between working and broken?
4. Understand dependencies - What does this code depend on?

Phase 3: Hypothesis and Testing

Apply the scientific method:

1. Formulate ONE clear hypothesis - "The error occurs because X"
2. Design minimal test - Change ONE variable at a time
3. Predict the outcome - What should happen if hypothesis is correct?
4. Run the test - Execute and observe
5. Verify results - Did it behave as predicted?
6. Iterate or proceed - Refine hypothesis if wrong, implement if right

Phase 4: Implementation

1. Create failing test case - Captures the bug behavior
2. Implement single fix - Address root cause, not symptoms
3. Verify test passes - Confirms fix works
4. Run full test suite - Ensure no regressions
5. If fix fails, STOP - Re-evaluate hypothesis

Critical rule: If THREE or more fixes fail consecutively, STOP. This signals architectural problems requiring discussion, not more patches.

Red Flags - Process Violations

Stop immediately if you catch yourself thinking:

• "Quick fix for now, investigate later"
• "One more fix attempt" (after multiple failures)
• "This should work" (without understanding why)
• "Let me just try..." (without hypothesis)
• "It works on my machine" (without investigating difference)

Warning Signs of Deeper Problems

Consecutive fixes revealing new problems in different areas indicates architectural issues:

• Stop patching
• Document what you've found
• Discuss with team before proceeding
• Consider if the design needs rethinking

Common Debugging Scenarios

Test Failures

1. Read the FULL error message and stack trace
2. Identify which assertion failed and why
3. Check test setup - is the test environment correct?
4. Check test data - are mocks/fixtures correct?
5. Trace to the source of unexpected value

Runtime Errors

1. Capture the full stack trace
2. Identify the line that throws
3. Check what values are undefined/null
4. Trace backward to find where bad value originated
5. Add validation at the source

"It worked before"

1. Use git bisect to find the breaking commit
2. Compare the change with previous working version
3. Identify what assumption changed
4. Fix at the source of the assumption violation

Intermittent Failures

1. Look for race conditions
2. Check for shared mutable state
3. Examine async operation ordering
4. Look for timing dependencies
5. Add deterministic waits or proper synchronization

Debugging Checklist

Before claiming a bug is fixed:

• Root cause identified and documented
• Hypothesis formed and tested
• Fix addresses root cause, not symptoms
• Failing test created that reproduces bug
• Test now passes with fix
• Full test suite passes
• No "quick fix" rationalization used
• Fix is minimal and focused

Success Metrics

Systematic debugging achieves ~95% first-time fix rate vs ~40% with ad-hoc approaches.

Signs you're doing it right:

• Fixes don't create new bugs
• You can explain WHY the bug occurred
• Similar bugs don't recur
• Code is better after the fix, not just "working"

Integration with Other Skills

• testing-patterns: Create test that reproduces the bug before fixing

Testing Patterns and Utilities

Testing Philosophy

Test-Driven Development (TDD):

• Write failing test FIRST
• Implement minimal code to pass
• Refactor after green
• Never write production code without a failing test

Behavior-Driven Testing:

• Test behavior, not implementation
• Focus on public APIs and business requirements
• Avoid testing implementation details
• Use descriptive test names that describe behavior

Factory Pattern:

• Create getMockX(overrides?: Partial<X>) functions
• Provide sensible defaults
• Allow overriding specific properties
• Keep tests DRY and maintainable

Test Utilities

Custom Render Function

Create a custom render that wraps components with required providers:

// src/utils/testUtils.tsx
import { render } from '@testing-library/react-native';
import { ThemeProvider } from './theme';

export const renderWithTheme = (ui: React.ReactElement) => {
  return render(
    <ThemeProvider>{ui}</ThemeProvider>
  );
};

Usage:

import { renderWithTheme } from 'utils/testUtils';
import { screen } from '@testing-library/react-native';

it('should render component', () => {
  renderWithTheme(<MyComponent />);
  expect(screen.getByText('Hello')).toBeTruthy();
});

Factory Pattern

Component Props Factory

import { ComponentProps } from 'react';

const getMockMyComponentProps = (
  overrides?: Partial<ComponentProps<typeof MyComponent>>
) => {
  return {
    title: 'Default Title',
    count: 0,
    onPress: jest.fn(),
    isLoading: false,
    ...overrides,
  };
};

// Usage in tests
it('should render with custom title', () => {
  const props = getMockMyComponentProps({ title: 'Custom Title' });
  renderWithTheme(<MyComponent {...props} />);
  expect(screen.getByText('Custom Title')).toBeTruthy();
});

Data Factory

interface User {
  id: string;
  name: string;
  email: string;
  role: 'admin' | 'user';
}

const getMockUser = (overrides?: Partial<User>): User => {
  return {
    id: '123',
    name: 'John Doe',
    email: 'john@example.com',
    role: 'user',
    ...overrides,
  };
};

// Usage
it('should display admin badge for admin users', () => {
  const user = getMockUser({ role: 'admin' });
  renderWithTheme(<UserCard user={user} />);
  expect(screen.getByText('Admin')).toBeTruthy();
});

Mocking Patterns

Mocking Modules

// Mock entire module
jest.mock('utils/analytics');

// Mock with factory function
jest.mock('utils/analytics', () => ({
  Analytics: {
    logEvent: jest.fn(),
  },
}));

// Access mock in test
const mockLogEvent = jest.requireMock('utils/analytics').Analytics.logEvent;

Mocking GraphQL Hooks

jest.mock('./GetItems.generated', () => ({
  useGetItemsQuery: jest.fn(),
}));

const mockUseGetItemsQuery = jest.requireMock(
  './GetItems.generated'
).useGetItemsQuery as jest.Mock;

// In test
mockUseGetItemsQuery.mockReturnValue({
  data: { items: [] },
  loading: false,
  error: undefined,
});

Test Structure

describe('ComponentName', () => {
  beforeEach(() => {
    jest.clearAllMocks();
  });

  describe('Rendering', () => {
    it('should render component with default props', () => {});
    it('should render loading state when loading', () => {});
  });

  describe('User interactions', () => {
    it('should call onPress when button is clicked', async () => {});
  });

  describe('Edge cases', () => {
    it('should handle empty data gracefully', () => {});
  });
});

Query Patterns

// Element must exist
expect(screen.getByText('Hello')).toBeTruthy();

// Element should not exist
expect(screen.queryByText('Goodbye')).toBeNull();

// Element appears asynchronously
await waitFor(() => {
  expect(screen.findByText('Loaded')).toBeTruthy();
});

User Interaction Patterns

import { fireEvent, screen } from '@testing-library/react-native';

it('should submit form on button click', async () => {
  const onSubmit = jest.fn();
  renderWithTheme(<LoginForm onSubmit={onSubmit} />);

  fireEvent.changeText(screen.getByLabelText('Email'), 'user@example.com');
  fireEvent.changeText(screen.getByLabelText('Password'), 'password123');
  fireEvent.press(screen.getByTestId('login-button'));

  await waitFor(() => {
    expect(onSubmit).toHaveBeenCalled();
  });
});

Anti-Patterns to Avoid

Testing Mock Behavior Instead of Real Behavior

// Bad - testing the mock
expect(mockFetchData).toHaveBeenCalled();

// Good - testing actual behavior
expect(screen.getByText('John Doe')).toBeTruthy();

Not Using Factories

// Bad - duplicated, inconsistent test data
it('test 1', () => {
  const user = { id: '1', name: 'John', email: 'john@test.com', role: 'user' };
});
it('test 2', () => {
  const user = { id: '2', name: 'Jane', email: 'jane@test.com' }; // Missing role!
});

// Good - reusable factory
const user = getMockUser({ name: 'Custom Name' });

Best Practices

1. Always use factory functions for props and data
2. Test behavior, not implementation
3. Use descriptive test names
4. Organize with describe blocks
5. Clear mocks between tests
6. Keep tests focused - one behavior per test

Running Tests

# Run all tests
npm test

# Run with coverage
npm run test:coverage

# Run specific file
npm test ComponentName.test.tsx

Integration with Other Skills

• react-ui-patterns: Test all UI states (loading, error, empty, success)
• systematic-debugging: Write test that reproduces bug before fixing