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prefer-type-fest-string-length

Require TypeFest StringLength over StringToArray<T>['length'].

Targeted pattern scopeโ€‹

This rule reports StringToArray<T>['length'] indexed-access type patterns and prefers the canonical StringLength<T> from type-fest.

What this rule reportsโ€‹

  • TSIndexedAccessType nodes of the shape StringToArray<T>['length'].
  • Direct, aliased, and namespace-qualified StringToArray references imported from type-fest.

Detection boundariesโ€‹

  • Reports StringToArray<T>['length'] with exactly one type argument forwarded.
  • Autofixes by replacing the indexed-access type with StringLength<T> and inserting a type-fest import when absent.
  • Does not autofix where the StringLength identifier is shadowed by a type parameter in scope.
  • Does not flag StringToArray<T, Options>['length']; StringToArray options can change non-literal string behavior.
  • Does not flag StringToArray<T>[0] or other non-'length' index expressions.
  • Does not flag local StringToArray helpers that are not imported from type-fest.
  • Does not flag StringToArray references shadowed by an enclosing type parameter.

Why this rule existsโ€‹

StringLength<T> is the canonical TypeFest utility for computing the length of a string type.

The pattern StringToArray<T>['length'] is equivalent only for the default one-argument StringToArray<T> form, but it exposes an implementation detail: turning the string into a tuple or array and then reading its length. StringLength<T> states the intent directly and keeps the type expression shorter.

โŒ Incorrectโ€‹

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

type EventNameLength = StringToArray<"user.created">["length"];

โœ… Correctโ€‹

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

type EventNameLength = StringLength<"user.created">;

Behavior and migration notesโ€‹

  • StringLength<S> returns the length of a literal string and number for non-literal strings.
  • StringLength<S> is implemented as StringToArray<S>['length'] in TypeFest, so the one-argument migration is safe.
  • StringToArray<S, { mapNonLiteralsDirectly: true }>['length'] is intentionally ignored because that option changes how non-literal string parts are represented.

ESLint flat config exampleโ€‹

import typefest from "eslint-plugin-typefest";

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

When not to use itโ€‹

Disable this rule if your codebase intentionally documents the intermediate StringToArray<T> representation and wants to keep the length extraction visually tied to that intermediate array form.

Package documentationโ€‹

TypeFest package documentation:

Source file: source/string-length.d.ts

/**
 * Returns the length of the given string.
 *
 * @example
 *  ```
 *  import type {StringLength} from 'type-fest';
 *
 *  type A = StringLength<'abcde'>;
 *  //=> 5
 *  ```;
 */

Rule catalog ID: R126

Further readingโ€‹

Adoption resourcesโ€‹