TS Debug

TypeScript Debugging

Not sure what's wrong? Start with the Node inspector — step through the code and observe. Once you can characterize the problem (slow, leaking memory, hanging), switch to the matching profiler.

Every Node flag above is stock — no build step, no loader, no dependency. why-is-node-running is the single gap worth a package: the built-in process.getActiveResourcesInfo() names the handle types holding the loop open but not where they were created. Run scripts/setup.sh to add it.

Behavioral Rules

• Lock the bug with a failing test once reproduced — before patching incorrect behavior, capture the faulty behavior as a regression test (through the public interface where one exists), so the fix is verified and the bug can't silently regress; skip this for pure performance or memory investigations, where a test through the public interface is rarely the right artifact
• Suggest only one tool per problem — don't list all options and ask the user to choose
• Explain the fit in one sentence before showing usage — e.g. "Since the process hangs after the work finishes, why-is-node-running is the right tool here."
• Prefer the inspector as the default for general debugging; only reach for profilers when the problem is clearly performance- or memory-related
• Reach for a built-in before a dependency — adding a package to a project mid-investigation is a cost the user pays forever, so justify it
• Run the TypeScript source, not the build — stack frames and breakpoints land on the right lines with no source maps; only a compiled node dist/… run needs --enable-source-maps
• Place the debugger statement just before the suspect line, not at the top of the function
• Debug tests in a single worker — --no-file-parallelism, otherwise breakpoints land in a worker thread the debugger isn't attached to
• Never leave instrumentation in production code paths — debugger statements, why-is-node-running dumps, and heap-snapshot calls must be removed before deploying

Running TypeScript directly

node file.ts erases type annotations in place and runs the result — no build step, no loader, no source maps. Because types are blanked rather than removed, every line number in a stack trace matches the source file exactly.

This runs unflagged on Node 22.18+ and 23.6+. Check node --version before recommending it: on anything older the same command needs --experimental-strip-types, and the whole table above assumes the source runs directly.

Type stripping only erases; it refuses TypeScript that has to emit runtime code:

SyntaxError [ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX]: TypeScript enum is not supported in strip-only mode

enum and namespace are the two that come up in practice. Rewriting them into erasable syntax (a const object, a plain module) is the durable fix — the escape hatch that used to cover them, --experimental-transform-types, was removed in Node 26. When rewriting isn't on the table, run the file through tsx instead.

Native execution also ignores paths in tsconfig — if a project maps import aliases, they won't resolve.

Tool Reference

Node inspector

Pauses on the first line and waits for a debugger to attach on port 9229. Open chrome://inspect in Chrome, or attach VS Code to the running process.

node --inspect-brk src/index.ts                      # TypeScript source
node --inspect-brk --enable-source-maps dist/index.js # compiled output

Set breakpoints in the DevTools/VS Code UI, or in code:

export function transfer(amount: number): void {
  debugger; // execution pauses here when an inspector is attached
  applyTransfer(amount);
}

VS Code attach configuration:

{
  "name": "Attach to process",
  "type": "node",
  "request": "attach",
  "port": 9229,
  "skipFiles": ["<node_internals>/**", "${workspaceFolder}/node_modules/**"]
}

Tip: inspector.open(9229, undefined, true) from node:inspector activates the inspector at runtime — use it when the interesting state only exists after startup.

vitest --inspect-brk

Debug a failing test the same way, with tests forced onto the main thread so breakpoints are hit.

pnpm exec vitest --inspect-brk --no-file-parallelism tests/index.test.ts

node --cpu-prof

CPU profiler. Writes a .cpuprofile to the working directory; load it in the Chrome DevTools Performance tab, which renders it both as a flame chart (where time goes across the call tree) and as a sortable function list (which single function is hot).

node --cpu-prof src/index.ts
# → CPU.<date>.<time>.<pid>.<tid>.<seq>.cpuprofile

Narrow the output with --cpu-prof-dir and --cpu-prof-name.

node --heap-prof + heap snapshots

--heap-prof shows which allocation sites the memory came from; a heap snapshot shows what is still retained. Use the profile to find the allocator, the snapshot to find the retainer.

node --heap-prof src/index.ts
# → Heap.<date>.<time>.<pid>.<tid>.<seq>.heapprofile

import v8 from "node:v8";

v8.writeHeapSnapshot("./before.heapsnapshot");
await runWorkload();
v8.writeHeapSnapshot("./after.heapsnapshot");

Load both .heapsnapshot files in the Chrome DevTools Memory tab and compare them to see what survived.

process.getActiveResourcesInfo (built-in)

The zero-dependency first look at a process that won't exit. Returns the types of the handles and requests keeping the event loop alive:

console.log(process.getActiveResourcesInfo()); // [ 'Timeout' ]

Often that's enough — a lone 'Timeout' or 'TTYWrap' points straight at the culprit. When the type alone doesn't identify which one, reach for the stacks.

why-is-node-running

Dumps the open handles and requests with the stack trace of where each was created — the part getActiveResourcesInfo() can't give you.

import whyIsNodeRunning from "why-is-node-running";

setTimeout(() => whyIsNodeRunning(), 5000); // prints every active handle with its stack

Import it when the investigation starts and delete the import when it ends.

console.dir

Use console.dir to replace ad-hoc console.log debugging when output is hard to read — nested objects collapsed to [Object], truncated arrays. It's not a debugger; use it when the problem is visibility of data, not stepping through logic.

console.dir(someNestedValue, { depth: null, colors: true });