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Stateless Components from Stateful Components

Passing State Change Functions as Props

If a React parent component defines a function that changes its state, that function can be passed to a child component and invoked within the child to update the parent’s state.

// Child component receives the function as a prop
function NameInput({ onNameChange, name }) {
  return (
    <div>
      <input onChange={onNameChange} />
      <p>{name}</p>
    </div>
  );
}


// Parent component owns state and passes down both state and setter function
function Name() {
  const [name, setName] = useState('');


  const handleNameChange = (e) => {
    setName(e.target.value);
  };


  return <NameInput onNameChange={handleNameChange} name={name} />;
}

Event Handlers and State in React

Event handler functions in React are often used to update state. These handler functions often receive an event as an argument, which is used to update state values correctly.

In the example code, we use event.target.value to get the input’s value.

function MyComponent() {
  const [ text, setText ] = useState("");


  const handleChange = (event) => {
    setText(event.target.value);
  }
  
  return (
    <div>
      <input onChange={handleChange} value={text} />
      <p>You typed {text}</p>
    </div>
  );
}

Stateful and Stateless Components

In React, a stateful component is a component that holds some state. Stateless components, by contrast, have no state. Note that both types of components can use props.

In the example, there are two React components. The Store component is stateful and the Week component is stateless.

function Store() {
  const [sell, setSell] = useState("anything");
  
  return (
    <div>
      <h1>I'm selling {sell}.</h1>
      <button onClick={() => setSell("books")}>Change Item</button>
    </div>
  );
}


function Week(props) {
  return <h1>Today is {props.day}!</h1>;
}

Parent State Pattern

A common React programming pattern involves using stateful parent components to manage state and pass it down to stateless child components as props. This approach centralizes state management in the parent while keeping child components focused on presentation and rendering.

Stateless child components receive data and event handlers through props, making them simpler to test and reuse. The parent component controls the state logic and updates, while children display the data they receive. This separation of concerns creates a clear data flow and makes component responsibilities easier to understand.

// Stateless child
function BabyYoda(props) {
  return <h2>I am {props.name}!</h2>;
}


// Stateful parent
function Yoda() {
  const [name, setName] = useState("Toyoda");
  
  return (
    <div>
      <BabyYoda name={name} />
      <button onClick={() => setName("Grogu")}>Reveal True Name</button>
    </div>
  );
}

Changing Props and State

In React, a component should never change its own props directly. A parent component should change them.

State, on the other hand, is the opposite of props: a component keeps track of its own state and can change it at any time.

The example code shows a component that accepts a prop, subtitle, which never changes. It also has a state object which does change.

function Clock(props) {
  const [ date, setDate ] = useState(new Date());


  const updateTime = () => {
    setDate(new Date());
  }
  
  return (
    <div>
      <h1>It is currently {date.toLocaleTimeString()}</h1>
      <h2>{props.subtitle}</h2>
      <button onClick={updateTime}>Update the clock</button>
    </div>
  );
}

Using Stateless Updaters and Presenters

A common React programming pattern is to use a parent stateful component to manage state and define state-updating methods. Then, it will render stateless child components.

One or more of those child components will be responsible for updating the parent state (via methods passed as props). One or more of those child components will be responsible for displaying that state.

In the example code, StatefulParent renders <InputComponent> to change its state and uses <DisplayComponent> to display it.

function StatefulParent() {
  const [value, setValue] = useState();
  


  const handler(event) => {
    // Update state here
  }


  
  turn (
    iv>
      nputComponent onChange={handler} />
      isplayComponent valueToDisplay={value} />
    div>
  );
  
}
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