react-doctor/js-hoist-intl

Hoist `new Intl.NumberFormat(...)` to module scope or wrap in `useMemo` — Intl constructors allocate dozens of objects per locale lookup

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

Validation prompt

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The rule fires on new Intl.X(...) where X is one of NumberFormat, DateTimeFormat, Collator, RelativeTimeFormat, ListFormat, PluralRules, Segmenter, or DisplayNames, AND the NewExpression sits inside a function body (FunctionDeclaration / FunctionExpression / ArrowFunctionExpression). Module-scope constructions are skipped. False positive when locale or options depend on per-call dynamic input that can't be cached.

Fix prompt

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Hoist to module scope: const numberFormatter = new Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' }); In React, when locale or options vary, wrap with useMemo(() => new Intl.NumberFormat(locale), [locale]). Each constructor loads and allocates locale-data tables that are expensive to rebuild per render or per list item. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl

Related rules

More Performance rules from the rules reference:

• react-doctor/js-hoist-regexp: Hoist `new RegExp(...)` (or large regex literals) to a module-level constant so it isn't recompiled on every loop iteration
• react-doctor/js-index-maps: Build an index `Map` once outside the loop instead of `array.find(...)` inside it
• 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