# E0230: TypeScript's 'readonly' feature is not allowed in JavaScript code

`readonly` on class properties is a TypeScript feature. It is a syntax error to use `readonly` in JavaScript code:

    class Fetcher {
      readonly baseURL;

      constructor(baseURL) {
        this.baseURL = baseURL;
      }
    }

To fix this error, erase the `readonly`:

    class Fetcher {
      baseURL;

      constructor(baseURL) {
        this.baseURL = baseURL;
      }
    }

Alternatively, rename your file to have a `.ts` or `.tsx` suffix

Introduced in quick-lint-js version 2.6.0.

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