prefer-readonly

Require private members to be marked as readonly if they're never modified outside of the constructor.

🔧

Some problems reported by this rule are automatically fixable by the --fix ESLint command line option.

💭

This rule requires type information to run.

Private member variables (whether using the private modifier or private # fields) are never permitted to be modified outside of their declaring class. If that class never modifies their value, they may safely be marked as readonly.

This rule reports on private members are marked as readonly if they're never modified outside of the constructor.

Flat Config

eslint.config.mjs

export default tseslint.config({
  rules: {
    "@typescript-eslint/prefer-readonly": "error"
  }
});

Legacy Config

.eslintrc.cjs

module.exports = {
  "rules": {
    "@typescript-eslint/prefer-readonly": "error"
  }
};

Try this rule in the playground ↗

Examples

❌ Incorrect

class Container {
  // These member variables could be marked as readonly
  private neverModifiedMember = true;
  private onlyModifiedInConstructor: number;
  #neverModifiedPrivateField = 3;

  public constructor(
    onlyModifiedInConstructor: number,
    // Private parameter properties can also be marked as readonly
    private neverModifiedParameter: string,
  ) {
    this.onlyModifiedInConstructor = onlyModifiedInConstructor;
  }
}

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✅ Correct

class Container {
  // Public members might be modified externally
  public publicMember: boolean;

  // Protected members might be modified by child classes
  protected protectedMember: number;

  // This is modified later on by the class
  private modifiedLater = 'unchanged';

  public mutate() {
    this.modifiedLater = 'mutated';
  }

  // This is modified later on by the class
  #modifiedLaterPrivateField = 'unchanged';

  public mutatePrivateField() {
    this.#modifiedLaterPrivateField = 'mutated';
  }
}

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Options

This rule accepts the following options:

type Options = [
  {
    /** Whether to restrict checking only to members immediately assigned a lambda value. */
    onlyInlineLambdas?: boolean;
  },
];

const defaultOptions: Options = [{ onlyInlineLambdas: false }];

onlyInlineLambdas

Whether to restrict checking only to members immediately assigned a lambda value. Default: false.

{
  "@typescript-eslint/prefer-readonly": [
    "error",
    { "onlyInlineLambdas": true },
  ],
}

Example of code for the { "onlyInlineLambdas": true } options:

❌ Incorrect

class Container {
  private onClick = () => {
    /* ... */
  };
}

Open in Playground

✅ Correct

class Container {
  private neverModifiedPrivate = 'unchanged';
}

Open in Playground

When Not To Use It

If you aren't trying to enforce strong immutability guarantees, this rule may be too restrictive for your project.

---

Type checked lint rules are more powerful than traditional lint rules, but also require configuring type checked linting.

See Troubleshooting > Linting with Type Information > Performance if you experience performance degradations after enabling type checked rules.

Resources

• Rule source
• Test source