What is the difference between props and state in React?

React has become one of the most popular libraries for building user interfaces, and if you're reading this on codedamn, chances are you're looking to understand the core principles of React better. Two fundamental concepts of React that every developer should understand are "props" and "state". Both play crucial roles in determining how a component renders and behaves. However, they serve different purposes and are used in different scenarios. Let's dive deep into each.

1. Basic Definitions

1.1 Props

Definition: Props, short for properties, are a way of passing data from parent to child components. They are read-only and help components receive values from their parents.

Immutability of props: In React, props are immutable, meaning once they are set by the parent component, they cannot be changed or modified by the child component. This immutable nature ensures predictable behavior of components.

1.2 State

Definition: State is a built-in object in a React component where you can store property values that belong to that component. It is primarily used to store the local state of a component.

Mutability of state: Unlike props, state is mutable. This means that it can be changed, and when it does change, the component re-renders to reflect those changes.

When and why to use state: State is used when a component has data that may change over time and should cause a re-render when it does. For example, if you're building a counter application, the counter's value would be stored in the state, and it would change each time the user clicks a button.

2. Key Differences between Props and State

2.1 Origin of Data

Props: Props are passed down from a parent component. It's a way for parents to communicate with their children and pass data to them.
State: The state is managed within the component itself. It's local to the component and determines the component's behavior based on data changes.

2.2 Mutability

Props: As mentioned, props are read-only. Once a parent component passes a prop to a child component, the child should not modify it.
State: State is mutable and designed to be changed, especially in response to user actions or other dynamic events.

2.3 Component Rerender

When props change: A component will re-render when its props change. This ensures that the component's UI stays consistent with the data it receives.
When state changes: A component will also re-render when its local state changes, allowing for dynamic behavior in response to data changes or user interactions.

2.4 Purpose and Usage

Props: The primary use of props is to pass data and callback functions to child components. It allows for a unidirectional data flow, where the parent dictates the values and behavior of the child component.
State: State is used to store local and temporary data that might change over time. For instance, user input, form data, or any dynamic data that doesn't come from the parent as a prop but affects the component's behavior or appearance.

In conclusion, while both props and state play pivotal roles in the React ecosystem, understanding their differences and when to use which is crucial. Remember, props are all about receiving data from parents and ensuring a one-way data flow, while state is about managing local, mutable data within a component. For more in-depth knowledge and examples, it's always a good idea to check the official React documentation.

3. Common Use Cases

3.1 Props

Props in React, short for "properties," are a mechanism to pass data from a parent component to its children. Let's look at common scenarios where props play a pivotal role.

Passing data to child components: One of the primary uses of props is to send data from parent to child. This ensures a unidirectional data flow, making it easier to track where data comes from.
```
function ParentComponent() {
  const data = "Hello from Parent!";
  return <ChildComponent message={data} />;
}
```
Callback functions to notify parent components of child events: Parents often need to know about events happening within their child components. By passing down callback functions as props, child components can notify parents about specific events.
```
function ParentComponent() {
  function handleChildClick() {
    console.log("Child clicked!");
  }
  return <ChildComponent onClick={handleChildClick} />;
}
```
Conditional rendering based on props: You can conditionally render parts of your component based on the props it receives.
```
function WelcomeMessage(props) {
  return props.show ? <h2>Welcome back!</h2> : null;
}
```

3.2 State

State allows React components to change their output over time in response to user actions, network responses, or anything else.

Form input handling: Managing form inputs is a typical use-case for state. Each input's value can be maintained in the state and then updated based on user interactions.

1class FormComponent extends React.Component {
2  state = {
3    inputValue: ""
4  };
5
6  handleInputChange = (event) => {
7    this.setState({ inputValue: event.target.value });
8  }
9
10  render() {
11    return <input value={this.state.inputValue} onChange={this.handleInputChange} />;
12  }
13}

Component lifecycle events: State is crucial when you need to load data from an API. For instance, you might have a loading state to manage UI display during data fetching.

Conditional rendering based on component’s internal conditions: Just like with props, you can conditionally render UI elements based on the state.

function ToggleComponent() {
  const [isVisible, setIsVisible] = React.useState(false);
  return (
    <>
      {isVisible && <div>Now you see me!</div>}
      <button onClick={() => setIsVisible(!isVisible)}>Toggle Visibility</button>
    </>
  );
}

4. The Lifecycle of Props and State

Changes in props or state lead a component to re-render. Understanding when and how this happens is key for optimized React applications.

Explanation of how and when props and state trigger renders: Whenever there's a change in a component's props or state, it leads to a re-render of that component and its children.
Component lifecycle methods and their interaction with props and state (for class components): Class components have lifecycle methods like componentDidMount, shouldComponentUpdate, and componentDidUpdate. These methods offer opportunities to act on component updates due to changes in props or state.
The effect of props and state on hooks (for functional components): For functional components, the useEffect hook can be utilized to perform side-effects in reaction to prop or state changes.

5. Best Practices

5.1 Props

Avoid prop drilling using context or state management solutions: Prop drilling is passing props from a parent component down multiple levels to a child component. Instead, use the Context API or state management libraries like Redux or MobX.
Validating props using PropTypes: PropTypes is a runtime type checking tool that can ensure the props a component receives are of the expected type.
```
Component.propTypes = {
  name: PropTypes.string.isRequired
};
```

5.2 State

Using the state in a controlled way: Always use this.setState in class components or the useState hook's update function in functional components to change state.
Avoiding direct mutations: Never modify state directly. Always return a new version of the state.
The importance of using functional updates when the next state depends on the previous state: Especially with hooks, if the next state is a derivative of the previous state, always use the functional form of the state updater.
```
const [count, setCount] = React.useState(0);
setCount(prevCount => prevCount + 1);
```

6. Advanced Concepts (Optional)

6.1 Higher-Order Components and Props Manipulation

Higher-order components (HOCs) are functions that take a component and return a new component with additional props or changed behavior. They can be used to manipulate or extend the props passed to the wrapped component.

6.2 Hooks: useState and useEffect in managing local state

useState and useEffect are foundational hooks in React. useState lets you manage local state in functional components, while useEffect is for managing side-effects.

6.3 Context API for global state management

The Context API allows you to share values across the component tree without explicitly passing them through each component. It's a handy tool for managing global state or theming.

Conclusion

Props and state are the heartbeats of any React application. While props allow parent-child component communication, state empowers components with dynamic data. Recognizing the fundamental differences between the two and their appropriate use-cases is paramount for effective React development.

Additional Resources

Official React documentation on props and state
We at codedamn recommend further readings on the React official documentation and exploring tutorials available on our platform.