dep.js

// @flow

const K = require('./main');

new K({errorHandler(err) { err.code; }});

main.js

// @flow

type Options = {
    errorHandler?: (err: Error) => void,
};

class K {
    constructor(options: Options) {
        }
}
module.exports = K;

/*
new K({
    errorHandler(err) {
          err.code;
            },
});
*/

output

> flow version
Flow, a static type checker for JavaScript, version 0.33.0
> flow check
dep.js:5
  5: new K({errorHandler(err) { err.code; }});
                                    ^^^^ property `code`. Property not found in
  5: new K({errorHandler(err) { err.code; }});
                                ^^^ Error


Found 1 error