Great work building out your first stateful function component in the last exercise. Just like you used the State Hook to manage a variable with string values, we can use the State Hook to manage the value of any primitive data type and even data collections like arrays and objects!

Have a look at the following function component. What data type does this state variable hold?

import React, { useState } from 'react';

function ToggleLoading() {
  const [isLoading, setIsLoading] = useState();

  return (
    <div>
      <p>The data is {isLoading ? 'Loading' : 'Not Loading'}</p>
      <button onClick={() => setIsLoading(true)}>
        Turn Loading On
      </button>
      <button onClick={() => setIsLoading(false)}>
        Turn Loading Off
      </button>
    </div>
  );
}

The ToggleLoading() function component above is using the simplest of all data types, a boolean. Booleans are frequently used in React applications to represent whether data has loaded or not. In the example above, we see that true and false values are passed to the state setter, setIsLoading(). This code works just fine as is, but what if we want our component to start off with isLoading set to true?

To initialize our state with any value we want, we simply pass the initial value as an argument to the useState() function call.

const [isLoading, setIsLoading] = useState(true);

There are three ways in which this code affects our component:

1. During the first render, the initial state argument is used.
2. When the state setter is called, React ignores the initial state argument and uses the new value.
3. When the component re-renders for any other reason, React continues to use the same value from the previous render.

If we don’t pass an initial value when calling useState(), then the current value of the state during the first render will be undefined. Often, this is perfectly fine for the machines, but it can be unclear to the humans reading our code. So, we prefer to explicitly initialize our state. If we don’t have the value needed during the first render, we can explicitly pass null instead of just passively leaving the value as undefined.