# E0348: unexpected '?' in type; use '| void' to make an optional type

Using a question mark as a prefix or suffix to make parameter types optional is invalid. Instead add a second 'void' type to the parameter.

    import * as fs from 'fs';

    const file = "./out.txt";
    const data = "test";

    fs.promises.writeFile(file, data)
      .then((err: Error?) => {
        if (err) throw err;
      });

To fix this warning, we need to replace the '?' suffix or prefix with the expression '| void'

    import * as fs from 'fs';

    const file = "./out.txt";
    const data = "test";

    fs.promises.writeFile(file, data)
      .then((err: Error | void) => {
        if (err) throw err;
      });

Documentation for other errors