The `mocha/prefer-arrow-callback` rule does not work as documented

[justinhelmer]

This is a great rule, but the documentation (which appears to just come from the original core rule) implies that arrow functions are preferred.

Instead, the mocha-aware rule discourages arrow functions. The reasoning totally resonates.

What I would have expected is, that the rule would behave as described by the documentation:

The following examples will not be flagged:

// arrow function callback
foo(a => a); // OK

... then, when used in combination with the allowUnboundThis rule, we could get the best of both worlds - arrow functions by default, but function expressions when this is used:

{ "allowUnboundThis": false } will flag the following examples:

foo(function() { this.a; });

foo(function() { (() => this); });

someArray.map(function(itm) { return this.doSomething(itm); }, someObject);

I tried disabling the no-mocha-arrows rule thinking that it might create a conflict, but still the --fix replaces my arrow functions with function expressions. I would expect (based on the documentation) that configuration like this:

module.exports = {
  env: {
    mocha: true,
  },
  extends: ['plugin:mocha/recommended'],
  rules: {
    'no-mocha-arrows': 'off',
    'prefer-arrow-callback': 'off',
    'mocha/prefer-arrow-callback': 'error',
  },
};

...would produce results that look like this:

describe('this is not used so it keeps the arrow fn', () => {
  beforeEach(function () {
    this.foo = true;
  });

  it('should keep the arrow fn', () => {
    expect(true).toBe(true);
  });

  it('should replace the arrow fn with a function expression', function () {
    expect(this.foo).toBe(true);
  });
});

Does this make sense?

Thanks for the great plugin. This is not a deal-breaker, just a bit more verbose than it needs to be since arrow functions can be preserved according to the core ESLint rule.