Memory Leak Debugging

Memory Leak Debugging

This skill provides expert guidance and workflows for finding, diagnosing, and fixing memory leaks in JavaScript and Node.js applications using Chrome DevTools MCP.

Core Principles

• Prefer MCP memory tools: Do NOT attempt to read raw .heapsnapshot files directly, as they are extremely large and will consume too many tokens. Use the Chrome DevTools MCP heap snapshot tools to summarize, compare, and inspect snapshots.
• Isolate the Leak: Determine if the leak is in the browser (client-side) or Node.js (server-side).
• Common Culprits: Look for detached DOM nodes, unhandled closures, global variables, event listeners not being removed, and caches growing unbounded. Note: Detached DOM nodes are sometimes intentional caches; always ask the user before nulling them.
• Close Loaded Snapshots: Heap snapshots can be large. After completing an investigation, use close_heapsnapshot for each loaded snapshot to release memory held by the MCP server.

Workflows

1. Capturing Snapshots

When investigating a frontend web application memory leak, utilize the chrome-devtools-mcp tools to interact with the application and take snapshots.

• Use tools like click, navigate_page, fill, etc., to manipulate the page into the desired state.
• Revert the page back to the original state after interactions to see if memory is released.
• Repeat the same user interactions 10 times to amplify the leak.
• Use take_heapsnapshot to save .heapsnapshot files to disk at baseline, target (after actions), and final (after reverting actions) states.

2. Comparing Snapshots

Once you have generated .heapsnapshot files using take_heapsnapshot, compare them with Chrome DevTools MCP memory tools.

• Start with get_heapsnapshot_summary for each snapshot to confirm that the files load and to compare high-level totals.
• Use compare_heapsnapshots to compare baseline and target snapshots. Start without classIndex for the summary diff, then request detailed class diffs only for suspicious growth.
• Use the summary output from compare_heapsnapshots before drilling into specific node IDs.

3. Inspecting Retainers

When a class or object type grows unexpectedly, inspect the retaining chain with the MCP tools before changing code.

• Use get_heapsnapshot_class_nodes to list instances of the suspicious class.
• Use get_heapsnapshot_retainers, get_heapsnapshot_retaining_paths, get_heapsnapshot_dominators, and get_heapsnapshot_edges to understand why representative nodes are still reachable.
• Use get_heapsnapshot_duplicate_strings when string growth dominates the diff.
• Read references/common-leaks.md for examples of common memory leaks and how to fix them after the retaining path points at application code.

4. External Tool Fallback

If the built-in MCP memory tools are not enough, use external tools as a fallback rather than reading raw snapshots directly.

• Read references/memlab.md for how to use memlab to analyze generated heap snapshots.
• If memlab is not available, use the fallback script in the references directory to compare two .heapsnapshot files and identify the top growing objects and common leak types.

Run the fallback script using Node.js:

node skills/memory-leak-debugging/references/compare_snapshots.js <baseline.heapsnapshot> <target.heapsnapshot>

The script will analyze and output the top growing objects by size and highlight the 3 most common types of memory leaks (for example, detached DOM nodes, closures, and contexts) if they are present.