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 tools.

Prerequisites

Advanced memory debugging tools (compare_heapsnapshots, get_heapsnapshot_details, etc.) are only available when the server is started with the --memoryDebugging flag. First check if these tools are available; if not, try to read the MCP configuration file to check if --memoryDebugging is enabled.

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 page-scoped tools like click, navigate_page, fill, etc. (specifying pageId) 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 (with pageId) 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 by specifying classIndex.
• Use the summary output from compare_heapsnapshots before drilling into specific node IDs.

3. Inspecting Retainers and Dominator Chains

When a class or object type grows unexpectedly, inspect the retaining chain and dominators 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_object_details with a specific nodeId to retrieve detailed object metadata (size, type, distance, and DOM detachedness).
• 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. Advanced Analysis and Categorized Filters

Use built-in MCP memory tools and filters to pinpoint specific leak categories directly without external tools.

• Use get_heapsnapshot_details or get_heapsnapshot_class_nodes with filterName to target common leak causes:
  • objectsRetainedByDetachedDomNodes: Identifies detached DOM elements retained in memory.
  • objectsRetainedByEventHandlers: Identifies objects kept alive by unremoved event listeners.
  • objectsRetainedByContexts: Identifies objects trapped in closures or execution contexts.
  • objectsRetainedByConsole: Identifies objects retained by console logging.