How would you persist expanded state in a file explorer?
Store expanded folder paths in a Set or object. Save to localStorage on every toggle. On initial render, read from localStorage and expand the folders that were previously expanded. This persists state across page refreshes.
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More FAQs in File Explorer Interview Tips and Common Questions
Start with the data structure (tree object). Write the recursive render function first. Test with a simple 2-level tree, then deeper. Add icons and CSS last. Handle empty folders. Get the recursion right before anything else.
Load children on expand, not on initial render. When a folder is expanded, fetch its children from an API. Show a loading indicator while fetching. This avoids loading the entire tree upfront for large directories.
Virtualization (only render visible nodes), lazy loading (load children on expand), and search/filter to narrow down. Do not render 10,000 DOM nodes at once. Use a virtual list library or implement custom virtualization.
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