react-doctor/js-index-maps

Build an index `Map` once outside the loop instead of `array.find(...)` inside it

• Category: Performance
• Severity: warn
• Source: oxlint-plugin-react-doctor
• Framework: global
• Enabled when: always

Validation prompt

Use this to decide whether a fired diagnostic is real or a false positive.

The rule fires on array.find(...) or array.findIndex(...) inside any loop body (uses createLoopAwareVisitors). False positive when the searched array changes per iteration (so a pre-built index can't be reused), or when the array is tiny (< ~10 items) where linear scan beats Map allocation and hashing overhead.

Fix prompt

Use this once validation confirms the diagnostic is real.

Before the loop, build const byId = new Map(items.map(item => [item.id, item])); then use byId.get(targetId) inside — O(1) lookups turn the total cost from O(n*m) into O(n+m). For multi-key lookups, build several Maps or a Map keyed on a composite string. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map

Related rules

More Performance rules from the rules reference:

• react-doctor/js-length-check-first: Short-circuit with `a.length === b.length && a.every((x, i) => x === b[i])` — unequal-length arrays exit immediately
• react-doctor/js-min-max-loop: Use `Math.min(...array)` / `Math.max(...array)` instead of sorting just to read the first or last element
• react-doctor/js-set-map-lookups: Use a `Set` or `Map` for repeated membership tests / keyed lookups — `Array.includes`/`find` is O(n) per call
• react-doctor/js-tosorted-immutable: Use `array.toSorted()` (ES2023) instead of `[...array].sort()` for immutable sorting without the spread allocation
• react-doctor/jsx-no-constructed-context-values: Memoize the context value with useMemo/useCallback or hoist it outside the render