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typescript/require-readonly-array-type-alias

Require readonly array-like types in type alias annotations.

Targeted pattern scope​

This rule targets top-level array-like type alias annotations, including:

  • T[]
  • tuple types ([A, B])
  • Array<T>
  • union/intersection members such as T[] | null

It checks only the type annotation attached to type aliases.

What this rule reports​

This rule reports type alias annotations that use mutable array-like types.

Why this rule exists​

Type aliases often define reusable API contracts. Using readonly array-like types in aliases helps propagate immutability expectations to all downstream uses.

❌ Incorrect​

type Names = string[];

type Pair = [string, number];

type Registry = Array<string>;

✅ Correct​

type Names = readonly string[];

type Pair = readonly [string, number];

type Registry = ReadonlyArray<string>;

Behavior and migration notes​

This rule is autofixable and also provides suggestions.

  • Array<T> is converted to ReadonlyArray<T>.
  • T[] and tuple types are converted to readonly ... forms.

The rule intentionally checks only top-level type alias annotations (and top-level union/intersection members), not nested object-property types.

Additional examples​

type Resolver = string[] | null;
// ❌ reported

type Resolver = readonly string[] | null;
// ✅ valid

type Config = { values: string[] };
// ✅ valid (nested property type is out of scope)

ESLint flat config example​

import etcMisc from "eslint-plugin-etc-misc";

export default [
 {
  plugins: { "etc-misc": etcMisc },
  rules: {
   "etc-misc/typescript/require-readonly-array-type-alias": "error",
  },
 },
];

When not to use it​

Disable this rule if your codebase intentionally uses mutable array type aliases or if you already enforce immutability through a broader rule strategy.

Package documentation​

Rule catalog ID: R117

Further reading​

Adoption resources​

  • Start at warning level in CI, then move to error after cleanup.
  • Use focused codemods/autofix batches per package or directory.