# E0311: missing parentheses around parameter; TypeScript optional parameter requires parentheses

In TypeScript, parameters can be explicitly marked as optional with `?`. It is a syntax error to write an arrow function with an optional parameter without parentheses around the parameter list:

    const error = message? => {
      throw new Error(message);
    };

To fix this error, write parentheses around the parameter list:

    const error = (message?) => {
      throw new Error(message);
    };

Introduced in quick-lint-js version 2.10.0.

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