withValidation.js

const withValidation = (state, field, value) => {
  const changedState = {
    ...state,
    fields: {
      ...state.fields,
      [field]: {
        ...state.fields[field],
        value,
      },
    },
  };

  return {
    ...changedState,
    isValid: isNewsletterFormModelValid(changedState),
  };
};