# E0260: TypeScript non-null assertion is not allowed on parameters

In TypeScript, `!` after an expression is a non-null assertion. It is a syntax error to write a non-null assertion after a function parameter:

    // color cannot be null.
    function colorToHexCode(color!: string): string {
      switch (color) {
        // ...
      }
    }

To fix this error, remove the `!` and ensure the parameter's type does not include `null`:

    // color cannot be null.
    function colorToHexCode(color: string): string {
      switch (color) {
        // ...
      }
    }

Introduced in quick-lint-js version 2.7.0.

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