Why is React not reactive?

React is not reactive in the traditional sense of reactive programming because it does not automatically track data changes and update the UI in response to any state changes without explicit calls.

In React, interface updates occur through calling the setState function or updating state with hooks (useState, useReducer), which triggers a re-render of the component. This is a declarative approach but not reactive in the sense of automatic data flow tracking and changes.

In reactive frameworks (e.g., Vue, Svelte, MobX), data changes are automatically tracked and lead to UI updates without the need to manually trigger updates.

Example in React:

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

  // UI update occurs only after calling setCount
  return <button onClick={() => setCount(count + 1)}>{count}</button>;
}

Here, React does not "react" to changes in the variable count but updates the UI only after calling setCount, which differs from reactive systems where changing a variable automatically reflects in the UI.