| 1 | # Coding Style |
| 2 | |
| 3 | ## Immutability (CRITICAL) |
| 4 | |
| 5 | ALWAYS create new objects, NEVER mutate: |
| 6 | |
| 7 | ```javascript |
| 8 | // WRONG: Mutation |
| 9 | function updateUser(user, name) { |
| 10 | user.name = name // MUTATION! |
| 11 | return user |
| 12 | } |
| 13 | |
| 14 | // CORRECT: Immutability |
| 15 | function updateUser(user, name) { |
| 16 | return { |
| 17 | ...user, |
| 18 | name |
| 19 | } |
| 20 | } |
| 21 | ``` |
| 22 | |
| 23 | ## File Organization |
| 24 | |
| 25 | MANY SMALL FILES > FEW LARGE FILES: |
| 26 | - High cohesion, low coupling |
| 27 | - 200-400 lines typical, 800 max |
| 28 | - Extract utilities from large components |
| 29 | - Organize by feature/domain, not by type |
| 30 | |
| 31 | ## Error Handling |
| 32 | |
| 33 | Business errors use `AppException + ResponseCode`, while protocol-layer errors may use standard `HttpException`. Do NOT wrap business logic with try-catch. |
| 34 | |
| 35 | ```typescript |
| 36 | // WRONG: try-catch + console + throw new Error |
| 37 | try { |
| 38 | const order = await this.orderService.getById(id) |
| 39 | } catch (error) { |
| 40 | console.error('Failed:', error) // ❌ console prohibited |
| 41 | throw new Error('Order not found') // ❌ use AppException |
| 42 | } |
| 43 | |
| 44 | // CORRECT: Let the framework handle exceptions |
| 45 | const order = await this.orderRepository.getById(id) |
| 46 | if (!order) { |
| 47 | throw new AppException(ResponseCode.OrderNotFound) |
| 48 | } |
| 49 | ``` |
| 50 | |
| 51 | Only use try-catch for infrastructure-level operations (external API calls, file I/O), and log with `this.logger.error()` instead of `console`. |
| 52 | |
| 53 | ## Input Validation |
| 54 | |
| 55 | ALWAYS validate user input: |
| 56 | |
| 57 | ```typescript |
| 58 | import { z } from 'zod' |
| 59 | |
| 60 | const schema = z.object({ |
| 61 | email: z.string().email(), |
| 62 | age: z.number().int().min(0).max(150) |
| 63 | }) |
| 64 | |
| 65 | const validated = schema.parse(input) |
| 66 | ``` |
| 67 | |
| 68 | ## Logger Error Format |
| 69 | |
| 70 | Error must be the first argument, message the second as a plain string: |
| 71 | |
| 72 | ```typescript |
| 73 | // CORRECT |
| 74 | this.logger.error(error, `Failed to process order ${orderId}`) |
| 75 | this.logger.warn(error, `Inventory check failed for product ${productId}`) |
| 76 | |
| 77 | // WRONG — error nested in object |
| 78 | this.logger.error({ path: 'xxx', message: 'yyy', error: err }) |
| 79 | |
| 80 | // WRONG — error interpolated into string template |
| 81 | this.logger.error(`Failed to process: ${error}`) |
| 82 | |
| 83 | // WRONG — message as object |
| 84 | this.logger.error(error, { context: 'xxx' }) |
| 85 | ``` |
| 86 | |
| 87 | Promise catch follows the same rule: |
| 88 | ```typescript |
| 89 | // CORRECT |
| 90 | promise.catch((err: Error) => this.logger.error(err, 'Failed to send notification')) |
| 91 | |
| 92 | // WRONG |
| 93 | promise.catch((err: unknown) => this.logger.error({ message: 'xxx', error: err })) |
| 94 | ``` |
| 95 | |
| 96 | ## Code Quality Checklist |
| 97 | |
| 98 | Before marking work complete: |
| 99 | - [ ] Code is readable and well-named |
| 100 | - [ ] Functions are small (<50 lines) |
| 101 | - [ ] Files are focused (<800 lines) |
| 102 | - [ ] No deep nesting (>4 levels) |
| 103 | - [ ] Proper error handling |
| 104 | - [ ] No console usage (use Logger instance) |
| 105 | - [ ] No hardcoded values |
| 106 | - [ ] No mutation (immutable patterns used) |
| 107 |