# E0238: assignment-asserted fields are not supported in interfaces

In TypeScript, `!` after a field name indicates a definite assignment assertion. These assertions only make sense for classes, not for interfaces. It is a syntax error to write a definite assignment assertion on an interface field:

    interface Point2D {
      x!: number;
      y!: number;
    }

To fix this error, remove the `!`:

    interface Point2D {
      x: number;
      y: number;
    }

Introduced in quick-lint-js version 2.6.0.

Documentation for other errors