prefer-type-fest-iterable-element

Require TypeFest IterableElement<T> over imported aliases like SetElement, SetEntry, and SetValues.

Targeted pattern scope

This rule focuses on imported legacy iterable helper aliases that TypeFest now models via IterableElement.

Type references that resolve to imported SetElement aliases.
Type references that resolve to imported SetEntry aliases.
Type references that resolve to imported SetValues aliases.

Unrelated type references and aliases from non-targeted imports are intentionally left untouched.

What this rule reports

This rule reports imported iterable helper aliases that should be consolidated to IterableElement.

Type references that resolve to imported SetElement aliases.
Type references that resolve to imported SetEntry aliases.
Type references that resolve to imported SetValues aliases.

Why this rule exists

IterableElement is the canonical TypeFest utility for extracting element types from iterable collections. Consolidating on one name makes collection type extraction patterns easier to audit and maintain.

❌ Incorrect

import type { SetElement } from "type-aliases";

type Value = SetElement<Set<string>>;

✅ Correct

import type { IterableElement } from "type-fest";

type Value = IterableElement<Set<string>>;

Behavior and migration notes

This rule targets imported alias names that duplicate IterableElement semantics (SetElement, SetEntry, SetValues).
Standardize on IterableElement<T> for sync and async iterable extraction patterns.
Keep legacy alias names only when external libraries expose them as a required public API.

Additional examples

❌ Incorrect — Additional example

import type { SetElement } from "type-aliases";

type Item = SetElement<Set<number>>;

✅ Correct — Additional example

import type { IterableElement } from "type-fest";

type Item = IterableElement<Set<number>>;

✅ Correct — Repository-wide usage

type StreamChunk = IterableElement<AsyncIterable<string>>;

ESLint flat config example

import typefest from "eslint-plugin-typefest";

export default [
 {
  plugins: { typefest },
  rules: {
   "typefest/prefer-type-fest-iterable-element": "error",
  },
 },
];

When not to use it

Disable this rule if compatibility requires preserving external alias names.

Package documentation

TypeFest package documentation:

Source file: source/iterable-element.d.ts

/**
 * Get the element type of an `Iterable`/`AsyncIterable`. For example, `Array`,
 * `Set`, `Map`, generator, stream, etc.
 *
 * This can be useful, for example, if you want to get the type that is yielded
 * in a generator function. Often the return type of those functions are not
 * specified.
 *
 * This type works with both `Iterable`s and `AsyncIterable`s, so it can be use
 * with synchronous and asynchronous generators.
 *
 * Here is an example of `IterableElement` in action with a generator function:
 *
 * @category Iterable
 *
 * @example
 *  ```
 *  import type {IterableElement} from 'type-fest';
 *
 *  function * iAmGenerator() {
 *  yield 1;
 *  yield 2;
 *  }
 *
 *  type MeNumber = IterableElement<ReturnType<typeof iAmGenerator>>;
 *  ```
 *
 *  And here is an example with an async generator:
 *
 * @example
 *  ```
 *  import type {IterableElement} from 'type-fest';
 *
 *  async function * iAmGeneratorAsync() {
 *  yield 'hi';
 *  yield true;
 *  }
 *
 *  type MeStringOrBoolean = IterableElement<ReturnType<typeof iAmGeneratorAsync>>;
 *  ```
 *
 *  Many types in JavaScript/TypeScript are iterables. This type works on all types that implement those interfaces.
 *
 *  An example with an array of strings:
 *
 * @example
 *  ```
 *  import type {IterableElement} from 'type-fest';
 *
 *  type MeString = IterableElement<string[]>;
 *  ```;
 *
 * @example
 *  ```
 *  import type {IterableElement} from 'type-fest';
 *
 *  const fruits = new Set(['🍎', '🍌', '🍉'] as const);
 *
 *  type Fruit = IterableElement<typeof fruits>;
 *  //=> '🍎' | '🍌' | '🍉'
 *  ```;
 */

Rule catalog ID: R042

prefer-type-fest-iterable-element

Targeted pattern scope

What this rule reports

Why this rule exists

❌ Incorrect

✅ Correct

Behavior and migration notes

Additional examples

❌ Incorrect — Additional example

✅ Correct — Additional example

✅ Correct — Repository-wide usage

ESLint flat config example

When not to use it

Package documentation

Further reading

Adoption resources

Targeted pattern scope​

What this rule reports​

Why this rule exists​

❌ Incorrect​

✅ Correct​

Behavior and migration notes​

Additional examples​

❌ Incorrect — Additional example​

✅ Correct — Additional example​

✅ Correct — Repository-wide usage​

ESLint flat config example​

When not to use it​

Package documentation​

Further reading​

Adoption resources​

Targeted pattern scope

What this rule reports

Why this rule exists

❌ Incorrect

✅ Correct

Behavior and migration notes

Additional examples

❌ Incorrect — Additional example

✅ Correct — Additional example

✅ Correct — Repository-wide usage

ESLint flat config example

When not to use it

Package documentation

Further reading

Adoption resources