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.

Documentation for other errors