Debug

debug

Conventions for the debug npm package.

Setup

import createDebug from 'debug';

// Server: include app/library name
const Debug = createDebug('MyApp:ClassName');

// Client: class name only
const Debug = createDebug('ClassName');

In Methods

async processOrder(orderId: string, items: Item[]) {
  const debug = Debug.extend('processOrder');
  debug('orderId %j', orderId);
  debug('items.length %j', items.length);  // Don't log large arrays

  const result = await this.orderService.create(orderId, items);
  debug('result %j', result);

  return result;
}

// Extend again inside callbacks when needed
items.forEach((item, index) => {
  const cbDebug = debug.extend(`item-${index}`);
  cbDebug('processing %j', item);
});

Rules

1. Blank line after debug statements - separates logging from logic

   // Good
   debug('orderId %j', orderId);
   debug('items.length %j', items.length);
   
   const result = await this.process(orderId);
   debug('result %j', result);
   
   return result;
   
   // Bad - debug mixed with logic
   debug('orderId %j', orderId);
   const result = await this.process(orderId);
   debug('result %j', result);
   return result;
   

2. One item per debug statement

   // Good
   debug('this.userId %j', this.userId);
   debug('this.token %j', this.token);
   
   // Bad
   debug('userId=%s token=%s', this.userId, this.token);
   

3. Keep labels simple - use variable name, not prose

   // Good
   debug('order %j', order);
   
   // Bad
   debug('The current order is: %j', order);
   

4. Label must match value - no transformations

   // Good - label matches value exactly
   debug('this.maxDate', this.maxDate);
   
   // Bad - label says maxDate but value is toISO() result
   debug('this.maxDate', this.maxDate.toISO());
   

5. Prose-only for flow markers - when there's no value to log

   debug('initialized');
   debug('enter pressed');
   debug('no loader registered');
   

6. Use %j for Server Side Only (usually)

   // Server: always use %j
   debug('config %j', config);
   
   // Client: don't use %j - browser console lets you inspect objects
   debug('config', config);
   
   // Exception: use %j client-side for timing issues where you need
   // to see object state at multiple points in time (otherwise you
   // only see the final state when you expand the object)
   

7. Log length for arrays - don't flood output

   debug('users.length %j', users.length);
   

8. Debug early returns - so you can trace why execution stopped

   if (!date) {
     debug('date %j', date);
   
     this.control.setValue(null);
     return;
   }
   

9. Debug inputs and outputs - trace execution flow

   async getUser(id: string) {
     const debug = Debug.extend('getUser');
     debug('id %j', id);
   
     const user = await this.db.findUser(id);
     debug('user %j', user);
   
     return user;
   }
   

10. Debug intermediate results - after each method call

const user = await this.userService.getUser(id);
debug('user %j', user);

const permissions = await this.authService.getPermissions(user.role);
debug('permissions %j', permissions);

const filtered = permissions.filter(p => p.active);
debug('filtered.length %j', filtered.length);

11. Debug loop iterations when needed

for (const item of items) {
  debug('item %j', item);
  // ... process item
}

12. Use console for always-on logging

console.log('Server started on port', port);  // Always show
console.error('Fatal error:', err);           // Always show
debug('request %j', req);                     // Only when DEBUG enabled

Enabling Debug Output

Server-side DEBUG strings can get long. Remember that wildcards work well:

# Full namespace
DEBUG=MyApp:OrderService:processOrder node app.js

# Wildcard - often sufficient
DEBUG=*processOrder node app.js
DEBUG=*Order* node app.js

Running Tests with Debug

Use the debug-test script to run tests with DEBUG output:

# Run specific test file with debug output
bash .claude/skills/debug/scripts/debug-test.sh '*OrderService*' -- src/order.spec.ts

# Run test by name pattern
bash .claude/skills/debug/scripts/debug-test.sh '*processOrder*' -- -t "should process order"

# Run all tests in a directory with debug
bash .claude/skills/debug/scripts/debug-test.sh 'MyApp:*' -- src/services/

The script sets the DEBUG environment variable and passes remaining arguments to npm test.