#no-instanceof-builtins

#Configuration

import { defineConfig, unicornPlugin } from '@rslint/core';

export default defineConfig([
  unicornPlugin.configs.recommended,
  {
    rules: {
      'unicorn/no-instanceof-builtins': 'error',
    },
  },
]);

#Rule Details

Disallow instanceof with built-in objects. instanceof can be unreliable across realms, so safer checks such as typeof, Array.isArray(), Object.prototype.toString.call(), or dedicated type helpers are preferred.

This rule can automatically fix Array, Function, and Error checks when useErrorIsError is enabled. Primitive wrapper checks such as String, Number, and Boolean provide suggestions instead of automatic fixes because converting object-wrapper values to typeof checks can change behavior.

Examples of incorrect code for this rule:

foo instanceof String;
foo instanceof Number;
foo instanceof Array;
foo instanceof Function;

Examples of correct code for this rule:

typeof foo === "string";
typeof foo === "number";
Array.isArray(foo);
typeof foo === "function";

Examples of incorrect code for this rule with { "strategy": "strict" }:

{ "unicorn/no-instanceof-builtins": ["error", { "strategy": "strict" }] }

foo instanceof Map;
foo instanceof Error;
foo instanceof Date;

Examples of incorrect code for this rule with { "include": ["HTMLElement"] }:

{ "unicorn/no-instanceof-builtins": ["error", { "include": ["HTMLElement"] }] }

foo instanceof HTMLElement;

Examples of correct code for this rule with { "exclude": ["String"] }:

{ "unicorn/no-instanceof-builtins": ["error", { "exclude": ["String"] }] }

foo instanceof String;

Examples of incorrect code for this rule with { "strategy": "strict", "useErrorIsError": true }:

{
  "unicorn/no-instanceof-builtins": [
    "error",
    { "strategy": "strict", "useErrorIsError": true }
  ]
}

foo instanceof Error;

#Options

#strategy

Type: "loose" | "strict"
Default: "loose"

• "loose" reports primitive wrapper constructors, Function, Array, and constructors listed in include.
• "strict" also reports built-in constructors such as Error, Map, Set, Date, typed arrays, Object, and RegExp.

#include

Type: string[]
Default: []

Additional constructor names to report.

#exclude

Type: string[]
Default: []

Constructor names to ignore. This takes precedence over strategy, include, and useErrorIsError.

#useErrorIsError

Type: boolean
Default: false

When enabled, Error checks are reported with an autofix to Error.isError(value).

#Original Documentation

• eslint-plugin-unicorn: no-instanceof-builtins
• Source code