Users report that your single-page app becomes slow and crashes after being open for a few hours. How do you investigate and fix it?

What the interviewer wants to hear

• Reproduce with a repeatable scenario and watch memory in the Performance monitor
• Compare heap snapshots before and after repeating an action to find growing objects
• Look at retainers to find what keeps detached DOM nodes or large objects alive
• Typical causes: missing effect cleanups, global listeners, timers, unbounded caches, third-party widgets
• Fix, add guards such as cache limits, and monitor memory or long tasks in production

Model answer

First I try to turn the report into a repeatable scenario, such as opening and closing a page twenty times, while watching the JS heap and DOM node count in the Performance monitor. If memory keeps climbing after garbage collection, it is a leak, not normal usage. I take a heap snapshot, repeat the action several times, take another, and use the comparison view to see which objects keep growing, often detached DOM elements or large arrays. The retainers panel shows the chain of references keeping them alive, which usually points to the cause: an effect without cleanup, a window or socket listener never removed, an interval still running, a module-level cache with no limit, or a chart or map widget that was never destroyed. I fix the root cause, add limits like an LRU cache size, and write a regression check if possible. In production I track long tasks and errors through RUM so we spot similar issues early.

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