# Task 2: Temperature

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This is the second of The 7 Tasks from 7GUIs (opens new window):

Challenges: bidirectional data flow, user-provided text input.

The task is to build a frame containing two textfields TC and TF representing the temperature in Celsius and Fahrenheit, respectively. Initially, both TC and TF are empty. When the user enters a numerical value into TC the corresponding value in TF is automatically updated and vice versa. When the user enters a non-numerical string into TC the value in TF is not updated and vice versa. The formula for converting a temperature C in Celsius into a temperature F in Fahrenheit is C = (F - 32) * (5/9) and the dual direction is F = C * (9/5) + 32.

Temperature Converter increases the complexity of Counter by having bidirectional data flow between the Celsius and Fahrenheit inputs and the need to check the user input for validity. A good solution will make the bidirectional dependency very clear with minimal boilerplate code.

Temperature Converter is inspired by the Celsius/Fahrenheit converter from the book Programming in Scala. It is such a widespread example—sometimes also in the form of a currency converter—that one could give a thousand references. The same is true for the Counter task.

# Modeling

Instead of thinking about this as bidirectional data flow, it can be simpler to think of this as a UI rendered from two values: C and F, and these two values can be updated due to events, such as CELSIUS for changing the C˚ input value and FAHRENHEIT for changing the F˚ input value. It just so happens that the <input> element both displays and updates the values, but that's just an implementation detail.

Note that when one of these events is sent to the machine, two things happen simultaneously:

  • The desired temperature value is assigned to the event value
  • The other temperature value is calculated and assigned based on that same event value


  • "active" - the state where converting the temperature is enabled


  • C - the temperature in degrees Celsius
  • F - the temperature in degrees Fahrenheit


  • "CELSIUS" - signals that the Celsius value should change
  • "FAHRENHEIT" - signals that the Fahrenheit value should change

# Coding

import { createMachine, assign } from 'xstate';

export const temperatureMachine = createMachine({
  initial: 'active',
  context: { C: undefined, F: undefined },
  states: {
    active: {
      on: {
        CELSIUS: {
          actions: assign({
            C: (_, event) => event.value,
            F: (_, event) =>
              event.value.length ? +event.value * (9 / 5) + 32 : ''
        FAHRENHEIT: {
          actions: assign({
            C: (_, event) =>
              event.value.length ? (+event.value - 32) * (5 / 9) : '',
            F: (_, event) => event.value

# Result