211231-ljharb-would-you-mutate-simple-example.js

function hasErrorsMutable(values) {
  const errors = {};
  if (values.foo === 'bar') {
    errors.foo = 'Foo has bar, no bueno';
  }
  if (values.bar === 'baz') {
    errors.bar = 'Bar has baz, no bueno';
  }
  return errors;
}

// Or

function hasErrorsImmutable(values) {
  let errors = {};
  if (values.foo === 'bar') {
    errors = { ...errors, foo: 'Foo has bar, no bueno' };
  }
  if (values.bar === 'baz') {
    errors = { ...errors, bar: 'Bar has baz, no bueno' };
  }
  return errors;
}