form.onChange(['Source field'], true)
  .setValue('Text field', state => {
    const [value] = state.changes;
    return value.text;
  });

form.onChange(['Rental price', 'Deposit'])
  .setValues(['Discount', 'Total'], state => {
    const [price, deposit] = state.changes;
    const discount = price.value * 0.07;
    const total = price.value + deposit.value - discount;
    return [discount, total];
  });

form.onChange(['Quantity'], true)
  .setVisibility(['Discount'], state => {
    const num = state.changes[0].value;
    return num > 99;
  });

form.onChange([''], true)
  .setVisibility (['Client'], state => {
    return state.currentStep < 2;
 });

form.onChange(['Catalog'])
  .setValue('Text', state => {
    const [item] = state.changes;
    return item.columns['Column'];
  });

form.onChange(['Start date', 'End date'])
  .validate('End date', state => {
    const [start, end] = state.changes;

    if (start.date && end.date && start.date >= end.date)
      return {
        errorMessage: 'The end date cannot be before the start date.'
      };

    return null;
  });

form.onChange(['Email', 'Phone'], true)
  .validate('Email', state => {
    const [email, phone] = state.changes;

    if (!email.text && !phone.text) 
      return {
        errorMessage: 'Add your email or phone number.'
      };

    return null;
  });

form.onChange(['Year', 'Number'])
  .validateAsync('Number', async state => {
    const [year, num] = state.changes;

    if (year.text && num.text) {
      const duplicates = await form.fetchSelfRegister(f => 
        f.fieldEquals('Year', year).fieldEquals('Number', num), 
        []
      );
      const firstDuplicate = duplicates.tasks[0];
      if (firstDuplicate)
        return {errorMessage: `Order <a href="#id${firstDuplicate.task_id}">has already been published.</a>`};
    }

    return null;
  });

form.onChange(['High priority', 'Date shifting'])
  .setValue('Due date', state => {
    const [checkmark, customDue] = state.changes;

    if (customDue.date)
      return {date: customDue.date};

    if (checkmark.checked)
      return {days_from_create: 1};

    return {days_from_create: 3};
  });

const orgId = 111111;

form.onChange([], true)
  .setVisibility(['Open / Done'], 
    state => false
  );

form.onChange([], true)
  .setStatus(state => {
    if (state.commenter.organization_id !== orgId)
      return {choice_name: 'In progress'};
  });

form.onChange(['Responsible'])
  .setStatus(state => {
    const [currVal] = state.changes;
    const [prevVal] = state.prev;

    if (!prevVal && currVal)
      return {choice_name: 'In progress'};
  });

form.onChange(['
PriorityThe name of a Catalog field in the service ticket form.
', '
ClientThe name of a Catalog field in the service ticket form.
'])
  .setValue('
Due dateThe name of a Date field in the service ticket form.
', 
    state => {
      const [priority, client] = state.changes;

      if (!priority || !client)
        return undefined;

      const tariff = client.columns['
Rate planThe Client catalog column of service rates.
'];
      const priorityValue = priority.columns['
PriorityThe name of a column in the Priority catalog.
'];
      const hours = getSlaHoursByTariffAndPriority(tariff, priorityValue);

      if (hours === undefined)
        return undefined;

      return {hours_from_create: hours};
    });

function getSlaHoursByTariffAndPriority(tariff, priority) {
  switch (tariff) {
    case '
BasicThe name of a rate plan from the Client catalog column.
':
      switch (priority) {
        case '
LowThe value from the Priority catalog column.
' : return 
64The deadline in hours.
;
        case '
MediumThe value from the Priority catalog column.
': return 
32Срок в часах
;
        case '
HighThe value from the Priority catalog column.
': return 
16The deadline in hours.
;
      }
    case '
StandardThe name of a rate plan from the Client catalog column.
':
      switch (priority) {
        case '
LowThe value from the Priority catalog column.
' : return 
32The deadline in hours.
;
        case '
MediumThe value from the Priority catalog column.
': return 
16Срок в часах
;
        case '
HighThe value from the Priority catalog column.
': return 
8The deadline in hours.
;
      }
    case '
PremiumThe name of a rate plan from the Client catalog column.
':
      switch (priority) {
        case '
LowThe value from the Priority catalog column.
' : return 
16The deadline in hours.
;
        case '
MediumThe value from the Priority catalog column.
': return 
8Срок в часах
;
        case '
HighThe value from the Priority catalog column.
': return 
4The deadline in hours.
;
      }
    default:
      return undefined;
  }
}

const emailPattern = /([a-z0-9-._]+)\@([a-z0-9-._]+)\.([a-z]{2,3})/ig;   

form.onChange(['Subject'], true)
  .setValue('Category', state => {
    const [currVal] = state.changes;
    const emails = (currVal.text || '').match(emailPattern);
    const first = emails ? emails[0] : '';
    return first;
  });

form.onChange(['Email'], true)
  .setVisibility(['Category'], state => {
    const [choice] = state.changes;
    return choice && choice.text === "mail@example.com";
  });

let catalogItems = null;

form.getCatalog("
US states and citiesThe catalog that binds values from two other catalogs.
").then(items => {
  catalogItems = items;
});

form.onChange(["
StateThe form field that defines values available for the City field.
"]).setFilter("
CityThe form field where values change depending on the value selected in the State field.
", state => {
  const [region] = state.changes;
  if (!catalogItems || !region ||
     !region.columns)
    return null;

  const regionCol = region.columns["
StateThe title of the column that defines values available in the City field.
"];
  const filtered = catalogItems
    .filter(item => item.columns["
StateThe title of the column that defines values available in the City field.
"] === regionCol)
    .map(item => item.columns["
CityThe name of the column where potential City fields come from.
"]);
  
  return filtered.length > 0
    ? {
        values: filtered
      }
    : null
});