25 lines
532 B
Markdown
25 lines
532 B
Markdown
---
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title: Use Set/Map for O(1) Lookups
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impact: LOW-MEDIUM
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impactDescription: O(n) to O(1)
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tags: javascript, set, map, data-structures, performance
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---
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## Use Set/Map for O(1) Lookups
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Convert arrays to Set/Map for repeated membership checks.
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**Incorrect (O(n) per check):**
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```typescript
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const allowedIds = ['a', 'b', 'c', ...]
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items.filter(item => allowedIds.includes(item.id))
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```
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**Correct (O(1) per check):**
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```typescript
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const allowedIds = new Set(['a', 'b', 'c', ...])
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items.filter(item => allowedIds.has(item.id))
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```
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