no-instanceof-array

Forbid use of instanceof Array in favor of Array.isArray(). This rule is part of eslint-plugin-modernization and ensures robust array detection in environments with multiple execution contexts.

Quick SummaryCopy Anchor Link

Why instanceof Array is UnreliableCopy Anchor Link

Problem: Each JavaScript environment (realm) has its own global objects, including Array. An array created in one iframe will not be an instance of the Array constructor in another iframe or the main window.

Risk: value instanceof Array will return false if value was created in a different realm, even if it is a valid array. This leads to subtle bugs where data processing logic is skipped or errors are thrown because an array wasn't recognized.

Error Message FormatCopy Anchor Link

The rule provides LLM-optimized error messages with actionable technical guidance:

⚠️ MODERNIZATION | Instanceof Array detected | MEDIUM
   Fix: Replace "value instanceof Array" with "Array.isArray(value)" for cross-realm compatibility | https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray

Message ComponentsCopy Anchor Link

Rule DetailsCopy Anchor Link

This rule flags an instanceof whose right-hand side is the built-in Array constructor, written Array or globalThis.Array. It is decided by binding: an Array the file declares itself (an import such as effect's Array module, a parameter, a local class) is not the built-in and is not reported. Array on the left of instanceof tests the constructor, not a value, and is not reported.

The allow option silences a file when one of its entries appears anywhere in the source text.

Why This MattersCopy Anchor Link

ConfigurationCopy Anchor Link

ExamplesCopy Anchor Link

❌ IncorrectCopy Anchor Link

// Fails for arrays from other realms (e.g., iframes)
if (value instanceof Array) {
  processItems(value);
}

const isArray = (v: any) => v instanceof Array;

✅ CorrectCopy Anchor Link

// Works correctly across all realms
if (Array.isArray(value)) {
  processItems(value);
}

const isArray = (v: any) => Array.isArray(v);

Related RulesCopy Anchor Link

• no-unsafe-type-narrowing - General type safety patterns
• require-array-type-guards - Ensuring reliable type checks

Further ReadingCopy Anchor Link

• Array.isArray() - MDN
• Cross-realm Array Comparison

Use this rule

npm install -D oxlint-plugin-modernization

{  "jsPlugins": ["oxlint-plugin-modernization"],  "rules": {    "modernization/no-instanceof-array": "error"  }}

import { defineConfig } from 'vite-plus';export default defineConfig({  lint: {    jsPlugins: ['oxlint-plugin-modernization'],    rules: {      'modernization/no-instanceof-array': 'error',    },  },});