What is the difference between state and props in React?

When building React applications, you’ll frequently encounter state and props—two core concepts that govern how data flows and how your UI updates. While both can hold data, they serve distinct purposes. Below is a concise comparison to clarify when and how to use each.

1. Data Flow and Ownership

Props

Data Flow: Props are passed down from a parent component to a child component.
Ownership: The parent “owns” the data and decides how and when to pass it down.

State

Data Flow: State is internal to a component, managed entirely within that component.
Ownership: The component “owns” its state. Other components don’t have direct access or control over it.

2. Mutability

Props

Read-Only: Props are immutable from the child’s perspective. The child cannot (and should not) modify its props directly.
Updates: If a parent re-renders and passes new props, the child sees updated props, but the child never mutates them.

State

Mutable: A component can update its own state using setState (class components) or state setter functions (hooks).
Updates Trigger Re-render: Changing state triggers a re-render, allowing the component (and often child components) to react to new data.

3. Typical Use Cases

Props

Passing Data Down: Sending user data, configuration options, or callbacks from a parent to a child.
Stateless Components: Components that only display or format data based on received props and don’t track any internal data changes.

State

Managing Internal Data: Tracking user inputs, toggles (like a modal open/close), or fetching data from APIs within the component.
Dynamic Updates: Needing to respond to user events or system events (e.g., form submissions, live updates).

4. Example

Props Example:

function ChildComponent({ message }) {
  return <p>{message}</p>; // read-only data from parent
}

function ParentComponent() {
  return <ChildComponent message="Hello from props!" />;
}

State Example:

function Counter() {
  const [count, setCount] = React.useState(0);

  return (
    <div>
      <p>{count}</p>
      <button onClick={() => setCount((prev) => prev + 1)}>
        Increment
      </button>
    </div>
  );
}

5. Best Practices

Keep State Local: If multiple components need the same data, consider lifting state up to a common ancestor or using context/global stores.
Minimize Props Drilling: For deeply nested data, use React Context or a global state manager like Redux, so you don’t have to pass props through multiple layers.

Strengthening Your React Skills

Understanding state vs. props is a fundamental step in writing React applications. For more advanced topics in React and to sharpen your JavaScript foundations, consider these courses from DesignGurus.io:

Grokking JavaScript Fundamentals
Deep dive into modern JavaScript, crucial for writing clean, efficient React code.
Grokking the Coding Interview: Patterns for Coding Questions
A structured way to master algorithmic thinking and data structures, essential for tackling high-level front-end or full-stack roles.

For hands-on practice and personalized feedback, check out Coding Mock Interviews, where ex-FAANG engineers help refine your approach and improve your interview readiness.

Conclusion

Props are passed down from parent to child, used for read-only data, and don’t trigger re-renders unless changed at the parent level.
State is managed internally by a component, can be updated, and any change triggers a re-render.

Knowing when to store data in state vs. pass it as props is central to writing predictable, maintainable React components. By properly distinguishing these two concepts, you’ll build clearer data flows and more efficient UIs in your React apps.

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