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Js Set Map Lookups

Source: .agents/references/coding-standard/vercel-react-best-practices/rules/js-set-map-lookups.md

Metadata​

  • title: Use Set/Map for O(1) Lookups
  • impact: LOW-MEDIUM
  • impactDescription: O(n) to O(1)
  • tags: javascript, set, map, data-structures, performance

Content​

Use Set/Map for O(1) Lookups​

Convert arrays to Set/Map for repeated membership checks.

Incorrect (O(n) per check):

const allowedIds = ['a', 'b', 'c', ...]
items.filter(item => allowedIds.includes(item.id))

Correct (O(1) per check):

const allowedIds = new Set(['a', 'b', 'c', ...])
items.filter(item => allowedIds.has(item.id))