Cursor rule

1---
2alwaysApply: false
3---
4 
5## Use flatMap to Map and Filter in One Pass
6 
7**Impact: LOW-MEDIUM (eliminates intermediate array)**
8 
9Chaining `.map().filter(Boolean)` creates an intermediate array and iterates twice. Use `.flatMap()` to transform and filter in a single pass.
10 
11**Incorrect (2 iterations, intermediate array):**
12 
13```typescript
14const userNames = users
15  .map(user => user.isActive ? user.name : null)
16  .filter(Boolean)
17```
18 
19**Correct (1 iteration, no intermediate array):**
20 
21```typescript
22const userNames = users.flatMap(user =>
23  user.isActive ? [user.name] : []
24)
25```
26 
27**More examples:**
28 
29```typescript
30// Extract valid emails from responses
31// Before
32const emails = responses
33  .map(r => r.success ? r.data.email : null)
34  .filter(Boolean)
35 
36// After
37const emails = responses.flatMap(r =>
38  r.success ? [r.data.email] : []
39)
40 
41// Parse and filter valid numbers
42// Before
43const numbers = strings
44  .map(s => parseInt(s, 10))
45  .filter(n => !isNaN(n))
46 
47// After
48const numbers = strings.flatMap(s => {
49  const n = parseInt(s, 10)
50  return isNaN(n) ? [] : [n]
51})
52```
53 
54**When to use:**
55- Transforming items while filtering some out
56- Conditional mapping where some inputs produce no output
57- Parsing/validating where invalid inputs should be skipped
58