eslint-plugin-is-kit

Type-aware ESLint rules for precise TypeScript predicates.

eslint-plugin-is-kit finds array predicates that are ambiguous, redundant, or weaker than a reusable type guard. It suggests predicates from is-kit only when they preserve runtime behavior and improve intent or type narrowing.

// Ambiguous: this also removes "", 0, false, and NaN when present.
values.filter(Boolean);

// Explicit: remove only null and undefined.
values.filter(isNotNil);

The plugin is separate from the zero-dependency is-kit runtime package and does not depend on it. Install is-kit when adopting predicates suggested by the stylistic rules.

Installation

Install the plugin alongside ESLint and typed-linting support:

pnpm add -D eslint-plugin-is-kit eslint typescript@^6.0.3 typescript-eslint@^8.69.0

Equivalent npm, yarn, and bun commands work as well.

If you enable the stylistic rules or use the suggested replacements, install is-kit as a runtime dependency:

pnpm add is-kit

Configuration

All rules require TypeScript type information. Enable typed linting and start with the correctness-focused recommended preset:

// eslint.config.js
import isKit from "eslint-plugin-is-kit";
import tseslint from "typescript-eslint";

export default [
  {
    files: ["**/*.ts"],
    ...isKit.configs.recommended,
    languageOptions: {
      parser: tseslint.parser,
      parserOptions: {
        projectService: true,
      },
    },
  },
];

Use strict to enable every rule:

export default [
  {
    files: ["**/*.ts"],
    ...isKit.configs.strict,
    languageOptions: {
      parser: tseslint.parser,
      parserOptions: {
        projectService: true,
      },
    },
  },
];

Presets

| Preset | Purpose | | --------------------------- | ----------------------------------------------- | | isKit.configs.recommended | Correctness rules suitable as a starting point. | | isKit.configs.stylistic | Opt-in preferences for reusable is-kit guards. | | isKit.configs.strict | Every correctness and stylistic rule. |

Individual rules

Rules can also be configured individually:

{
  plugins: { "is-kit": isKit },
  rules: {
    "is-kit/prefer-is-non-nullish": "warn",
  },
}

CommonJS

The package includes a CommonJS build. Its default export is available through the generated module namespace:

// eslint.config.cjs
const { default: isKit } = require("eslint-plugin-is-kit");

Rules

| Rule | What it reports | Recommended | | ------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------- | ----------- | | no-ambiguous-filter-boolean | filter(Boolean) when nullish and other falsy values can both be removed. | Yes | | no-redundant-predicate | An is-kit filter predicate that already accepts every array element. | Yes | | prefer-is-non-nullish | Inline nullish-removal filters that can use isNotNil. | No | | prefer-type-guard | Equivalent inline checks in filter, find, findLast, and every. | No |

The rules are deliberately conservative. They inspect built-in array methods, use TypeScript's type information, and skip cases where a replacement could change runtime behavior or weaken useful narrowing. They do not replace native checks in ordinary control flow.

No rule currently provides an autofix. Adding or changing imports is an intentional user decision, and filter(Boolean) can sometimes mean “keep only truthy values.”

Compatibility

| Dependency | Supported versions | | --------------------------- | --------------------------------- | | Node.js | ^22.13.0 or >=24.0.0 | | ESLint | ^8.57.0, ^9.0.0, or ^10.0.0 | | TypeScript | >=5.7.0 <6.1.0 | | @typescript-eslint/parser | ^8.69.0 |

The package ships ESM and CommonJS builds with declarations for both entry points.

Development

pnpm install
pnpm check
pnpm smoke

pnpm smoke packs the package, installs the tarball in an isolated consumer project, and verifies its imports, declarations, and ESLint behavior.

License

MIT