prefer-type-fest-array-values

Require TypeFest ArrayValues<T> over typeof values[number] constant array value extraction.

Targeted pattern scope

This is a type-aware rule. It targets indexed-access type queries whose object type is a typeof query that resolves to an array or tuple.

What this rule reports

typeof values[number] when values resolves to an array or tuple type.

The rule intentionally leaves non-typeof T[number] patterns to prefer-type-fest-array-element.

Why this rule exists

ArrayValues<T> documents the common constant-array pattern directly: derive a union from the values available in a runtime array or tuple.

❌ Incorrect

const statuses = ["queued", "running"] as const;

type Status = typeof statuses[number];

✅ Correct

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

const statuses = ["queued", "running"] as const;

type Status = ArrayValues<typeof statuses>;

Behavior and migration notes

This rule requires type information.
It only reports typeof values[number] when values resolves to an array-like type.
It does not report number-indexed object maps.
It does not report plain T[number]; use prefer-type-fest-array-element for that pattern.

Additional examples

❌ Incorrect — Additional example

declare const weekdays: readonly string[];

type WeekdayName = typeof weekdays[number];

✅ Correct — Additional example

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

declare const weekdays: readonly string[];

type WeekdayName = ArrayValues<typeof weekdays>;

✅ Correct — Non-targeted usage

type Statuses = readonly ["queued", "running"];

type Status = Statuses[number];

ESLint flat config example

import typefest from "eslint-plugin-typefest";

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

When not to use it

Disable this rule if you prefer native indexed-access syntax for value extraction from runtime arrays, or if your project does not use type-aware linting.

Package documentation

TypeFest package documentation:

Source file: source/array-values.d.ts

export type ArrayValues<T extends readonly unknown[]> = T[number];

Rule catalog ID: R110

prefer-type-fest-array-values

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 — Non-targeted 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 — Non-targeted 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 — Non-targeted usage

ESLint flat config example

When not to use it

Package documentation

Further reading

Adoption resources