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prefer-ts-extras-assert-present

Require assertPresent from ts-extras over manual == null throw guards.

Targeted pattern scopeโ€‹

This rule focuses on throw-only nullish guards that map directly to assertPresent(value).

Matched patternsโ€‹

  • if (value == null) { throw ... }
  • if (value === null || value === undefined) { throw ... }
  • if (value === undefined || value === null) { throw ... }

Only if statements that have no else branch and a throw-only consequent are reported.

Detection boundariesโ€‹

  • โœ… Reports nullish guards written as == null or explicit null/undefined OR checks.
  • โŒ Does not report a null-only guard (value === null) or undefined-only guard.
  • โŒ Does not report branches that do more than throw.
  • โŒ Does not auto-fix.

These boundaries keep matching deterministic and avoid broad semantic overreach during migration.

What this rule reportsโ€‹

Throw-only nullish guard blocks that can be replaced with assertPresent(value).

Why this rule existsโ€‹

assertPresent() communicates nullish-assertion intent and provides a reusable narrowing helper.

This is a high-signal utility for request handlers and parsing layers where nullable inputs are common.

โŒ Incorrectโ€‹

if (payload == null) {
 throw new Error("Missing payload");
}

โœ… Correctโ€‹

assertPresent(payload);

Behavior and migration notesโ€‹

  • assertPresent(value) narrows value to NonNullable<T>.
  • The rule covers nullish guards (== null or explicit null || undefined) with throw-only consequents.
  • Null-only or undefined-only assertions are intentionally left untouched.

Additional examplesโ€‹

โŒ Incorrect โ€” Additional exampleโ€‹

if (input === null || input === undefined) {
 throw new TypeError("input is required");
}

โœ… Correct โ€” Additional exampleโ€‹

assertPresent(input);

โœ… Correct โ€” Repository-wide usageโ€‹

assertPresent(currentUser);
assertPresent(sessionId);

ESLint flat config exampleโ€‹

import typefest from "eslint-plugin-typefest";

export default [
 {
  plugins: { typefest },
  rules: {
   "typefest/prefer-ts-extras-assert-present": "error",
  },
 },
];

When not to use itโ€‹

Disable this rule if your domain code requires custom error payloads inline at each nullish guard.

Package documentationโ€‹

ts-extras package documentation:

Source file: source/assert-present.ts

/**
 * Assert that the given value is present (non-nullable), meaning it is neither
 * `null` nor `undefined`.
 *
 * If the value is not present (`undefined` or `null`), a helpful `TypeError`
 * will be thrown.
 *
 * @category Type guard
 *
 * @example
 *  ```
 *  import {assertPresent} from 'ts-extras';
 *
 *  const unicorn = 'unicorn';
 *  assertPresent(unicorn);
 *
 *  const notUnicorn = null;
 *  assertPresent(notUnicorn);
 *  //=> TypeError: Expected a present value, got `null`
 *  ```
 */

Rule catalog ID: R013

Further readingโ€‹

Adoption resourcesโ€‹