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typescript/require-readonly-array-property-type

Require readonly array-like types for top-level property type annotations.

Targeted pattern scope​

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

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

It checks property signatures in interfaces and type literals.

What this rule reports​

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

Why this rule exists​

Property signatures define shared object contracts. Using readonly array-like property types communicates immutability intent and helps prevent accidental mutation through references that are passed around the codebase.

❌ Incorrect​

interface Config {
 values: string[];
}

type ApiConfig = {
 values: Array<string> | null;
};

✅ Correct​

interface Config {
 values: readonly string[];
}

type ApiConfig = {
 values: ReadonlyArray<string> | null;
};

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 property annotations (and top-level union/intersection members), not nested object-property types.

This rule has no options.

Additional examples​

interface Config {
 values: Promise<string[]>;
}
// ✅ valid (nested generic type is out of scope)

type Settings = {
 values: { nested: 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-property-type": "error",
  },
 },
];

When not to use it​

Disable this rule if your team intentionally models property arrays as mutable or if broader immutability rules already enforce your preferred constraints.

Package documentation​

Rule catalog ID: R142

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.