Best steps to debug a JavaScript heap out of memor

问题:

The Error:

So I get something that looks like this:

<--- Last few GCs --->

[53206:0x104800000]    14400 ms: Mark-sweep 854.6 (956.2) -> 854.6 (916.7) MB, 47.2 / 0.0 ms  (average mu = 0.871, current mu = 0.000) last resort GC in old space requested
[53206:0x104800000]    14445 ms: Mark-sweep 854.6 (916.7) -> 854.6 (915.2) MB, 44.3 / 0.0 ms  (average mu = 0.754, current mu = 0.000) last resort GC in old space requested

 <--- JS stacktrace --->

 ==== JS stack trace =========================================

     0: ExitFrame [pc: 0x32ff55ecfc7d]
 Security context: 0x04eea581d9b1 <JSObject>
     1: byteLength(aka byteLength) [0x4ee3fcd6129] [buffer.js:522] [bytecode=0x4ee55266c61 offset=204](this=0x04ee450825b1 <undefined>,0x04ee72d252e1 <Very long string[433772571]>,0x04eea587f519 <String[4]: utf8>)
     2: arguments adaptor frame: 3->2
     3: fromString(aka fromString) [0x4ee3fcef989] [buffer.js:333] [bytecode=0x4ee552620b9 offset=77](this=0x04...

 FATAL ERROR: CALL_AND_RETRY_LAST Allocation failed - JavaScript heap out of memory
  1: 0x1000645d3 node::Abort() [/usr/local/Cellar/node/11.12.0/bin/node]
  2: 0x100064c5b node::OnFatalError(char const*, char const*) [/usr/local/Cellar/node/11.12.0/bin/node]
  3: 0x10017e40f v8::Utils::ReportOOMFailure(v8::internal::Isolate*, char const*, bool) [/usr/local/Cellar/node/11.12.0/bin/node]
  4: 0x10017e3b0 v8::internal::V8::FatalProcessOutOfMemory(v8::internal::Isolate*, char const*, bool) [/usr/local/Cellar/node/11.12.0/bin/node]
  5: 0x100440714 v8::internal::Heap::UpdateSurvivalStatistics(int) [/usr/local/Cellar/node/11.12.0/bin/node]
  6: 0x10044681e v8::internal::Heap::SetUp() [/usr/local/Cellar/node/11.12.0/bin/node]
  7: 0x1004269af v8::internal::Factory::AllocateRawWithImmortalMap(int, v8::internal::PretenureFlag, v8::internal::Map*, v8::internal::AllocationAlignment) [/usr/local/Cellar/node/11.12.0/bin/node]
  8: 0x100428511 v8::internal::Factory::NewRawTwoByteString(int, v8::internal::PretenureFlag) [/usr/local/Cellar/node/11.12.0/bin/node]
  9: 0x100505f5e v8::internal::String::SlowFlatten(v8::internal::Isolate*, v8::internal::Handle<v8::internal::ConsString>, v8::internal::PretenureFlag) [/usr/local/Cellar/node/11.12.0/bin/node]
 10: 0x1001921f1 v8::String::Utf8Length(v8::Isolate*) const [/usr/local/Cellar/node/11.12.0/bin/node]
 11: 0x10004cd13 node::Buffer::(anonymous namespace)::ByteLengthUtf8(v8::FunctionCallbackInfo<v8::Value> const&) [/usr/local/Cellar/node/11.12.0/bin/node]
 12: 0x1001e2ac4 v8::internal::FunctionCallbackArguments::Call(v8::internal::CallHandlerInfo*) [/usr/local/Cellar/node/11.12.0/bin/node]
 13: 0x1001e2124 v8::internal::MaybeHandle<v8::internal::Object> v8::internal::(anonymous namespace)::HandleApiCallHelper<false>(v8::internal::Isolate*, v8::internal::Handle<v8::internal::HeapObject>, v8::internal::Handle<v8::internal::HeapObject>, v8::internal::Handle<v8::internal::FunctionTemplateInfo>, v8::internal::Handle<v8::internal::Object>, v8::internal::BuiltinArguments) [/usr/local/Cellar/node/11.12.0/bin/node]
 14: 0x1001e1944 v8::internal::Builtin_Impl_HandleApiCall(v8::internal::BuiltinArguments, v8::internal::Isolate*) [/usr/local/Cellar/node/11.12.0/bin/node]
 15: 0x32ff55ecfc7d

and I have absolutely no idea how to even start debugging this. I've looked at similar questions "Javascript heap out of memory" and understand that I can increase the memory limit (which seams to be the most commonly suggested solution).

---

The Code

The function that is causing the error is an express controller looking something like this (simplified):

const foo = async (req,res) => {
   // get users in chat room
   const users = await db.getUsers(req.query.roomId);
   res.send(users);
}

The error came when I added another user to the room. I've also checked that the error disappears if I filter out the last added user. From print-debugging I've also seen that all code executes up to the res.send statement so it seems the error is happening in that function.

---

The Question

The reason I'm hesitant at just adding more memory is that I'm just pushing the problem further away and instead of crashing at 6 users it will crash at 15 or 30 users.

So I would like to debug the script (perhaps see how much memory the users object takes up and how different forms of optimisations would affect that.

I simply don't know how to debug these kind of low level errors and wonder if there are some standard you can go through when encountering this kind of error?

---

PS: Just found it kinda funny that the top suggested similar question is:

How to create an HTML button that acts like a link?