E0228: TypeScript optional properties are not allowed in JavaScript code

? on class properties is a TypeScript feature. It is a syntax error to write ? on a property in JavaScript code:

class Entity {
  parent? = null;
}

To fix this error, erase the ?:

class Entity {
  parent = null;
}

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

Introduced in quick-lint-js version 2.6.0.

Documentation for other errors