**Custom hooks** allow us to reuse stateful logic across multiple components. To create a custom hook, we simply create a function starting with `use`. The easiest example of a custom hook is creating a reusable toggle state:


```js
function useToggle(initialState = false) {
  const [state, setState] = useState(initialState);
  const toggleFunction = () => setState(state => !state);
  return [state, toggleFunction];
}

function Main(props) { 
  const [ state, toggle ] = useToggle();
  return ( 
    <button onClick={toggle}>
      {state ? "Turn Off" : "Turn On"}
    </button>
  );
}
```

In the above code, we create our custom Hook function called `useToggle`. We create a state variable inside that function and assign a value of `false` (by default) to that state variable, apart from that, we also create an inline function that can be used for toggling the state variable's value. 

We return both of these in an array where the first value is the state variable's reference, and the second is the function. 

Using this custom hook, we can now create multiple independent togglable states in different (or the same) component(s) with ease:

```js
function useToggle(initialState = false) {
  const [state, setState] = useState(initialState);
  const toggleFunction = () => setState(state => !state);
  return [state, toggleFunction];
}


function OneButton(props) {
  const [ state, toggle ] = useToggle();
  return ( 
    <button onClick={toggle}>
      {state ? "Turn Off" : "Turn On"}
    </button>
  );
}

function TwoButtons(props) {
  const [ state_1, toggle_1 ] = useToggle();
  const [ state_2, toggle_2 ] = useToggle();
  return ( 
    <div>
      <button onClick={toggle_1}>
        {state_1 ? "Turn Off" : "Turn On"}
      </button>
      <button onClick={toggle_2}>
        {state_2 ? "Turn Off" : "Turn On"}
      </button>
    </div>
  );
}

function Main(props) { 
  const [ state, toggle ] = useToggle();
  return ( 
    <div>
      <TwoButtons />
      <OneButton />
    </div>
  );
}
```


As you can see, each button can be independently turned off/on :


In this section we will explore why React is useful and why it is worth learning it at all. We will also learn one prerequisite, ES6, which will be used throughout the course.

In this section, we will learn another prerequisite - JSX which will be used throughout the course. Finally, we will explore some example code to get acquainted with React.

In this section we will learn about the essential and practical parts of React excluding the components – which will be covered in the next section. We will look at how JSX can be used with React, and how to create and render more complex elements in the React DOM using JSX. We will also learn how to set up a React based project, and we will explore its structure for deepening our understanding.

In this section you’ll be introduced to React components. We will look at what React components are and why we need them. We will explore the two different types of React components and learn the syntax for creating them. We will use them to create a single page application that displays X.

In this section we will learn more about how to combine JavaScript and JSX to create dynamic content. We will also learn how to pass data between components. In the end we will also explore event handling.

In the end of the previous chapter, we tried making a Todo application though in our attempt to do so we soon discovered that we needed some method for re-rendering the components once the data is updated in a component as React wasn’t automatically doing that for us. In this section we will learn how to fix such issues using ‘Hooks’. We will learn and implement hooks in detail. By the end of this section, you will be able to create stateful components in React and combine them to create complex web applications.

In this chapter we will learn how to add different pages to our React application and render different components based on URL. You will gain sufficient knowledge to be able to create multi-page applications.