QReact · Interview preparation
When does React preserve component state?
Understand how position, component type, and keys determine whether React keeps or resets state.
The idea to remember.
React associates state with a component’s position and identity in the rendered tree. Replacing its component type or changing its key causes React to treat it as a different component and reset local state. Stable keys help preserve identity when list items move or update.
01Learn by doing
From first example to real project.
Start with the smallest working idea, examine a more detailed example, then look at a real application pattern. Adapt dependencies, error handling and data models to your project.
Basic: a stable list
01 / BEGINNERA stable ID lets React keep each row's identity.
const todos = [{ id: 1, text: 'Read docs' }, { id: 2, text: 'Build app' }];
function TodoList() {
return <ul>{todos.map(todo => <li key={todo.id}>{todo.text}</li>)}</ul>;
}Intermediate: intentional reset
02 / INTERMEDIATEChanging a key tells React this is a new form, so its local state resets.
function ProfileEditor({ person }) {
return <ProfileForm key={person.id} person={person} />;
}Real scenario: reorderable tasks
03 / REAL SCENARIOUse database IDs, not array positions, when tasks can move.
function TaskBoard({ tasks, onMove }) {
return tasks.map(task => (
<TaskCard key={task.id} task={task} onMove={() => onMove(task.id)} />
));
}02Check your understanding
Try it in your own words.
Explain when does React preserve component state without looking at the code. Then modify the intermediate example, describe one trade-off and identify when the real-world pattern fits.
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