Creating the countdown block feature

Introduction

Now we covered the component's basics, it's time to implement the countdown effectively. For that, we need to use a React hook called useState.

The `useState` hook

It is called within the functional component to update and consume the component state. The state represents the component's current state.

The useState returns a pair: the current state value and a function to update it.

Seeing the example provided on the previous step we can understand these concepts:

const [count, setCount] = useState(0)

In the above code piece, you might observe three things:

In the count variables, it's possible to get the current state;
setCount is a function used for updating it;
0 is its initial state;

const [timeRemaining, setTime] = useState<TimeSplit>({
  hours: '00', 
  minutes: '00', 
  seconds: '00'
})

Activity

First, we need to import a few functions and types to continue. Inside the Countdown component, import the following:
```
//react/Countdown.tsx
import React, { useState } from 'react'
import { TimeSplit } from './typings/global'
import { tick, getTwoDaysFromNow } from './utils/time'
```
The getTwoDaysFromNow function will be used to deal with edge cases. It'll be explained later on in this step.

Next step is to add the state update hook (useState):

//react/Countdown.tsx
const Countdown: StorefrontFunctionComponent<CountdownProps> = ({ targetDate }) => {
+   const [timeRemaining, setTime] = useState<TimeSplit>({
+     hours: '00',
+     minutes: '00',
+     seconds: '00'
+   })

    return (
      <div>
        { targetDate }
      </div>
    ) 
}

After doing that, we'll add a default constant targetDate for the edge case where the prop is not defined. We'll use as fallback a date that is defined as two days from the current date, this date is calculated on an util function that was previously imported from the /utils folder.
```
//react/Countdown.tsx
const DEFAULT_TARGET_DATE = getTwoDaysFromNow()
```

Now, we need to add the tick function and the DEFAULT_TARGET_DATE constant to make the countdown work:

//react/Countdown.tsx
const Countdown: StorefrontFunctionComponent<CountdownProps> = ({ targetDate = DEFAULT_TARGET_DATE }) => {
  const [timeRemaining, setTime] = useState<TimeSplit>({
    hours: '00',
    minutes: '00',
    seconds: '00'
})

+ tick(targetDate, setTime)

  return (
    <div>
      { targetDate }
    </div>
  ) 
}

At last but not least, change the h1 so that it shows the countdown that we've created. For that, we need to use the timeRemaining current state:

//react/Countdown.tsx
const Countdown: StorefrontFunctionComponent<CountdownProps> = ({ targetDate = DEFAULT_TARGET_DATE }) => {
  const [timeRemaining, setTime] = useState<TimeSplit>({
    hours: '00',
    minutes: '00',
    seconds: '00'
  })

  tick(targetDate, setTime)

  return (
    <div>   
-     <h1>{ targetDate }</h1>
+     <h1>{ `${timeRemaining.hours}:${timeRemaining.minutes}:${timeRemaining.seconds}` }</h1>
    </div>
  ) 
}

The countdown string formatting is in a HH:MM:SS format, made through an hours, minutes and seconds splitting.

Therefore, with these changes, we'll see a real-time update of the countdown! The result on the homepage is this:

ivansavev / store-block