State machine advent: Guard state transitions, guard actions (14/24)
Tags: javascript, typescript, react
Published: 2019-12-14 17:09:05

One really powerful concept of statecharts that we haven't covered yet are guards. They are conditional functions that determine whether a state transition should be taken, or actions executed. They are fundamental building blocks when building applications using statecharts. Today, we will cover how to use them.


# Guard actions

Let's say we want our thermostat to only work with temperatures below 100°C. We can do so by conditionally executing the action that changes our temperature in `context`. In XState, we can define this guard within events using the `cond` keyword. Its value is a function that will be invoked with the context, event, and must always return a boolean value.

```typescript
import { Machine, assign } = 'xstate';

const thermostatMachine = Machine({
  id: 'thermostat',
  initial: 'inactive',
  context: {
    temperature: 20,
  },
  states: {
    inactive: {
      on: {
        POWER_TOGGLE: 'active'
      }
    },
    active: {
      on: {
        POWER_TOGGLE: 'inactive',
        SET_TEMPERATURE: {
          cond: (context, event) => event.temperature < 100,
          actions: assign({
            temperature: (context, event) => event.temperature,
          }),
        }
      }
    },
  }
});
```

In plain English, our code above is saying

> Once the SET_TEMPERATURE event is called, evaluate the guard. If the temperature of our event is below 100°C, update the temperature of the machine. Should the temperature be higher than 99°C, do not perform any action

For a visual representation, you can even see the guard being visualized by the XState visualizer [here](https://xstate.js.org/viz/?gist=9b74b623527864848a7dadca836747d8) 🎉


# Guard state transitions

Likewise, should the guard evaluate to `false`, no state transitions will be taken. In the example above, no state transition was taken as no `target` was defined but let's say we only want our thermostat to be turned off if the temperature has risen to at least 10°C. Below that point, the thermostat should always actively be monitoring the temperature. 

```typescript
import { Machine, assign } = 'xstate';

const thermostatMachine = Machine({
  id: 'thermostat',
  initial: 'inactive',
  context: {
    temperature: 20,
  },
  states: {
    inactive: {
      on: {
        POWER_TOGGLE: 'active'
      }
    },
    active: {
      on: {
        POWER_TOGGLE: {
          cond: (context, event) => {
            const isWarmEnough = context.temperature >= 10;
            if (!isWarmEnough) console.log("I think it's too cold for you to turn this off");
            // Do not forget to return the boolean :) 
            return isWarmEnough;
          },
          target: 'inactive',
        },
        SET_TEMPERATURE: {
          cond: (context, event) => event.temperature < 100,
          actions: assign({
            temperature: (context, event) => event.temperature,
          }),
        }
      }
    },
  }
});
```

As seen above, only if the temperature is equal or greater than 10°C, the thermostat will go into the `inactive` state.

___

We've now seen how to guard transitions and actions using conditional logic. Guards are very important and especially when using them in conjunction with some other concepts - I'm eager to introduce the upcoming days - guards enable us to safely model a lot of complex behavior. I'm excited to be showing you what one can do with them. 

### About this series
Throughout the first 24 days of December, I'll publish a small blog post each day teaching you about the ins and outs of state machines and statecharts.

The first couple of days will be spent on the fundamentals before we'll progress to more advanced concepts.