QReact · Interview preparation
How does React decide whether two components are the same?
React considers a component's type and position in the tree, with keys providing explicit identity among siblings.
The idea to remember.
React considers a component's type and position in the tree, with keys providing explicit identity among siblings. Changing identity can reset local state, even if the new screen looks similar.
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 how does React decide whether two components are the same 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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