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Bun

Node.js module

v8

The 'node:v8' module provides access to V8-specific APIs, such as heap snapshots, code event notifications, and object statistics. It is used for advanced performance tuning and memory profiling.

Methods include v8.getHeapSnapshot(), v8.writeHeapSnapshot(), and v8.takeHeapSnapshot().

Works in Bun

`writeHeapSnapshot` and `getHeapSnapshot` are implemented. `serialize` and `deserialize` use JavaScriptCore's wire format, not V8's. Most other V8-specific methods are not implemented. For profiling, consider using `bun:jsc`.

  • class DefaultDeserializer

    A subclass of Deserializer corresponding to the format written by DefaultSerializer.

    • Reads the underlying wire format version. Likely mostly to be useful to legacy code reading old wire format versions. May not be called before .readHeader().

    • readDouble(): number;

      Read a JS number value. For use inside of a custom deserializer._readHostObject().

    • readHeader(): boolean;

      Reads and validates a header (including the format version). May, for example, reject an invalid or unsupported wire format. In that case, an Error is thrown.

    • length: number
      ): Buffer;

      Read raw bytes from the deserializer's internal buffer. The length parameter must correspond to the length of the buffer that was passed to serializer.writeRawBytes(). For use inside of a custom deserializer._readHostObject().

    • readUint32(): number;

      Read a raw 32-bit unsigned integer and return it. For use inside of a custom deserializer._readHostObject().

    • readUint64(): [number, number];

      Read a raw 64-bit unsigned integer and return it as an array [hi, lo] with two 32-bit unsigned integer entries. For use inside of a custom deserializer._readHostObject().

    • readValue(): any;

      Deserializes a JavaScript value from the buffer and returns it.

    • id: number,
      arrayBuffer: ArrayBuffer
      ): void;

      Marks an ArrayBuffer as having its contents transferred out of band. Pass the corresponding ArrayBuffer in the serializing context to serializer.transferArrayBuffer() (or return the id from serializer._getSharedArrayBufferId() in the case of SharedArrayBuffers).

      @param id

      A 32-bit unsigned integer.

      @param arrayBuffer

      An ArrayBuffer instance.

  • class DefaultSerializer

    A subclass of Serializer that serializes TypedArray(in particular Buffer) and DataView objects as host objects, and only stores the part of their underlying ArrayBuffers that they are referring to.

    • Returns the stored internal buffer. This serializer should not be used once the buffer is released. Calling this method results in undefined behavior if a previous write has failed.

    • id: number,
      arrayBuffer: ArrayBuffer
      ): void;

      Marks an ArrayBuffer as having its contents transferred out of band. Pass the corresponding ArrayBuffer in the deserializing context to deserializer.transferArrayBuffer().

      @param id

      A 32-bit unsigned integer.

      @param arrayBuffer

      An ArrayBuffer instance.

    • value: number
      ): void;

      Write a JS number value. For use inside of a custom serializer._writeHostObject().

    • writeHeader(): void;

      Writes out a header, which includes the serialization format version.

    • buffer: TypedArray
      ): void;

      Write raw bytes into the serializer's internal buffer. The deserializer will require a way to compute the length of the buffer. For use inside of a custom serializer._writeHostObject().

    • value: number
      ): void;

      Write a raw 32-bit unsigned integer. For use inside of a custom serializer._writeHostObject().

    • hi: number,
      lo: number
      ): void;

      Write a raw 64-bit unsigned integer, split into high and low 32-bit parts. For use inside of a custom serializer._writeHostObject().

    • val: any
      ): boolean;

      Serializes a JavaScript value and adds the serialized representation to the internal buffer.

      This throws an error if value cannot be serialized.

    • Reads the underlying wire format version. Likely mostly to be useful to legacy code reading old wire format versions. May not be called before .readHeader().

    • readDouble(): number;

      Read a JS number value. For use inside of a custom deserializer._readHostObject().

    • readHeader(): boolean;

      Reads and validates a header (including the format version). May, for example, reject an invalid or unsupported wire format. In that case, an Error is thrown.

    • length: number
      ): Buffer;

      Read raw bytes from the deserializer's internal buffer. The length parameter must correspond to the length of the buffer that was passed to serializer.writeRawBytes(). For use inside of a custom deserializer._readHostObject().

    • readUint32(): number;

      Read a raw 32-bit unsigned integer and return it. For use inside of a custom deserializer._readHostObject().

    • readUint64(): [number, number];

      Read a raw 64-bit unsigned integer and return it as an array [hi, lo] with two 32-bit unsigned integer entries. For use inside of a custom deserializer._readHostObject().

    • readValue(): any;

      Deserializes a JavaScript value from the buffer and returns it.

    • id: number,
      arrayBuffer: ArrayBuffer
      ): void;

      Marks an ArrayBuffer as having its contents transferred out of band. Pass the corresponding ArrayBuffer in the serializing context to serializer.transferArrayBuffer() (or return the id from serializer._getSharedArrayBufferId() in the case of SharedArrayBuffers).

      @param id

      A 32-bit unsigned integer.

      @param arrayBuffer

      An ArrayBuffer instance.

  • class GCProfiler

    This API collects GC data in current thread.

    • start(): void;

      Start collecting GC data.

    • Stop collecting GC data and return an object. The content of object is as follows.

      {
  "version": 1,
  "startTime": 1674059033862,
  "statistics": [
    {
      "gcType": "Scavenge",
      "beforeGC": {
        "heapStatistics": {
          "totalHeapSize": 5005312,
          "totalHeapSizeExecutable": 524288,
          "totalPhysicalSize": 5226496,
          "totalAvailableSize": 4341325216,
          "totalGlobalHandlesSize": 8192,
          "usedGlobalHandlesSize": 2112,
          "usedHeapSize": 4883840,
          "heapSizeLimit": 4345298944,
          "mallocedMemory": 254128,
          "externalMemory": 225138,
          "peakMallocedMemory": 181760
        },
        "heapSpaceStatistics": [
          {
            "spaceName": "read_only_space",
            "spaceSize": 0,
            "spaceUsedSize": 0,
            "spaceAvailableSize": 0,
            "physicalSpaceSize": 0
          }
        ]
      },
      "cost": 1574.14,
      "afterGC": {
        "heapStatistics": {
          "totalHeapSize": 6053888,
          "totalHeapSizeExecutable": 524288,
          "totalPhysicalSize": 5500928,
          "totalAvailableSize": 4341101384,
          "totalGlobalHandlesSize": 8192,
          "usedGlobalHandlesSize": 2112,
          "usedHeapSize": 4059096,
          "heapSizeLimit": 4345298944,
          "mallocedMemory": 254128,
          "externalMemory": 225138,
          "peakMallocedMemory": 181760
        },
        "heapSpaceStatistics": [
          {
            "spaceName": "read_only_space",
            "spaceSize": 0,
            "spaceUsedSize": 0,
            "spaceAvailableSize": 0,
            "physicalSpaceSize": 0
          }
        ]
      }
    }
  ],
  "endTime": 1674059036865
}

      Here's an example.

      import { GCProfiler } from 'node:v8';
const profiler = new GCProfiler();
profiler.start();
setTimeout(() => {
  console.log(profiler.stop());
}, 1000);

    • Returns the stored internal buffer. This serializer should not be used once the buffer is released. Calling this method results in undefined behavior if a previous write has failed.

    • id: number,
      arrayBuffer: ArrayBuffer
      ): void;

      Marks an ArrayBuffer as having its contents transferred out of band. Pass the corresponding ArrayBuffer in the deserializing context to deserializer.transferArrayBuffer().

      @param id

      A 32-bit unsigned integer.

      @param arrayBuffer

      An ArrayBuffer instance.

    • value: number
      ): void;

      Write a JS number value. For use inside of a custom serializer._writeHostObject().

    • writeHeader(): void;

      Writes out a header, which includes the serialization format version.

    • buffer: TypedArray
      ): void;

      Write raw bytes into the serializer's internal buffer. The deserializer will require a way to compute the length of the buffer. For use inside of a custom serializer._writeHostObject().

    • value: number
      ): void;

      Write a raw 32-bit unsigned integer. For use inside of a custom serializer._writeHostObject().

    • hi: number,
      lo: number
      ): void;

      Write a raw 64-bit unsigned integer, split into high and low 32-bit parts. For use inside of a custom serializer._writeHostObject().

    • val: any
      ): boolean;

      Serializes a JavaScript value and adds the serialized representation to the internal buffer.

      This throws an error if value cannot be serialized.

  • The promiseHooks interface can be used to track promise lifecycle events.

  • The v8.startupSnapshot interface can be used to add serialization and deserialization hooks for custom startup snapshots.

    node --snapshot-blob snapshot.blob --build-snapshot entry.js
    # This launches a process with the snapshot
    node --snapshot-blob snapshot.blob

    In the example above, entry.js can use methods from the v8.startupSnapshot interface to specify how to save information for custom objects in the snapshot during serialization and how the information can be used to synchronize these objects during deserialization of the snapshot. For example, if the entry.js contains the following script:

    'use strict';

import fs from 'node:fs';
import zlib from 'node:zlib';
import path from 'node:path';
import assert from 'node:assert';

import v8 from 'node:v8';

class BookShelf {
  storage = new Map();

  // Reading a series of files from directory and store them into storage.
  constructor(directory, books) {
    for (const book of books) {
      this.storage.set(book, fs.readFileSync(path.join(directory, book)));
    }
  }

  static compressAll(shelf) {
    for (const [ book, content ] of shelf.storage) {
      shelf.storage.set(book, zlib.gzipSync(content));
    }
  }

  static decompressAll(shelf) {
    for (const [ book, content ] of shelf.storage) {
      shelf.storage.set(book, zlib.gunzipSync(content));
    }
  }
}

// __dirname here is where the snapshot script is placed
// during snapshot building time.
const shelf = new BookShelf(__dirname, [
  'book1.en_US.txt',
  'book1.es_ES.txt',
  'book2.zh_CN.txt',
]);

assert(v8.startupSnapshot.isBuildingSnapshot());
// On snapshot serialization, compress the books to reduce size.
v8.startupSnapshot.addSerializeCallback(BookShelf.compressAll, shelf);
// On snapshot deserialization, decompress the books.
v8.startupSnapshot.addDeserializeCallback(BookShelf.decompressAll, shelf);
v8.startupSnapshot.setDeserializeMainFunction((shelf) => {
  // process.env and process.argv are refreshed during snapshot
  // deserialization.
  const lang = process.env.BOOK_LANG || 'en_US';
  const book = process.argv[1];
  const name = `${book}.${lang}.txt`;
  console.log(shelf.storage.get(name));
}, shelf);

    The resulted binary will get print the data deserialized from the snapshot during start up, using the refreshed process.env and process.argv of the launched process:

    BOOK_LANG=es_ES node --snapshot-blob snapshot.blob book1
    # Prints content of book1.es_ES.txt deserialized from the snapshot.

    Currently the application deserialized from a user-land snapshot cannot be snapshotted again, so these APIs are only available to applications that are not deserialized from a user-land snapshot.

  • function cachedDataVersionTag(): number;

    Returns an integer representing a version tag derived from the V8 version, command-line flags, and detected CPU features. This is useful for determining whether a vm.Script cachedData buffer is compatible with this instance of V8.

    console.log(v8.cachedDataVersionTag()); // 3947234607
// The value returned by v8.cachedDataVersionTag() is derived from the V8
// version, command-line flags, and detected CPU features. Test that the value
// does indeed update when flags are toggled.
v8.setFlagsFromString('--allow_natives_syntax');
console.log(v8.cachedDataVersionTag()); // 183726201
  • function deserialize(
    buffer: ArrayBufferView
    ): any;

    Uses a DefaultDeserializer with default options to read a JS value from a buffer.

    @param buffer

    A buffer returned by serialize.

  • detailLevel?: 'brief' | 'detailed'
    ): object;

    It returns an object with a structure similar to the cppgc::HeapStatistics object. See the V8 documentation for more information about the properties of the object.

    // Detailed
({
  committed_size_bytes: 131072,
  resident_size_bytes: 131072,
  used_size_bytes: 152,
  space_statistics: [
    {
      name: 'NormalPageSpace0',
      committed_size_bytes: 0,
      resident_size_bytes: 0,
      used_size_bytes: 0,
      page_stats: [{}],
      free_list_stats: {},
    },
    {
      name: 'NormalPageSpace1',
      committed_size_bytes: 131072,
      resident_size_bytes: 131072,
      used_size_bytes: 152,
      page_stats: [{}],
      free_list_stats: {},
    },
    {
      name: 'NormalPageSpace2',
      committed_size_bytes: 0,
      resident_size_bytes: 0,
      used_size_bytes: 0,
      page_stats: [{}],
      free_list_stats: {},
    },
    {
      name: 'NormalPageSpace3',
      committed_size_bytes: 0,
      resident_size_bytes: 0,
      used_size_bytes: 0,
      page_stats: [{}],
      free_list_stats: {},
    },
    {
      name: 'LargePageSpace',
      committed_size_bytes: 0,
      resident_size_bytes: 0,
      used_size_bytes: 0,
      page_stats: [{}],
      free_list_stats: {},
    },
  ],
  type_names: [],
  detail_level: 'detailed',
});

    // Brief
({
  committed_size_bytes: 131072,
  resident_size_bytes: 131072,
  used_size_bytes: 128864,
  space_statistics: [],
  type_names: [],
  detail_level: 'brief',
});
    @param detailLevel

    Default: 'detailed'. Specifies the level of detail in the returned statistics. Accepted values are:

    • 'brief': Brief statistics contain only the top-level allocated and used memory statistics for the entire heap.
    • 'detailed': Detailed statistics also contain a break down per space and page, as well as freelist statistics and object type histograms.

  • Get statistics about code and its metadata in the heap, see V8 GetHeapCodeAndMetadataStatistics API. Returns an object with the following properties:

    {
  code_and_metadata_size: 212208,
  bytecode_and_metadata_size: 161368,
  external_script_source_size: 1410794,
  cpu_profiler_metadata_size: 0,
}
  • function getHeapSnapshot(
    options?: HeapSnapshotOptions

    Generates a snapshot of the current V8 heap and returns a Readable Stream that may be used to read the JSON serialized representation. This JSON stream format is intended to be used with tools such as Chrome DevTools. The JSON schema is undocumented and specific to the V8 engine. Therefore, the schema may change from one version of V8 to the next.

    Creating a heap snapshot requires memory about twice the size of the heap at the time the snapshot is created. This results in the risk of OOM killers terminating the process.

    Generating a snapshot is a synchronous operation which blocks the event loop for a duration depending on the heap size.

    // Print heap snapshot to the console
import v8 from 'node:v8';
const stream = v8.getHeapSnapshot();
stream.pipe(process.stdout);
    @returns

    A Readable containing the V8 heap snapshot.

  • Returns statistics about the V8 heap spaces, i.e. the segments which make up the V8 heap. Neither the ordering of heap spaces, nor the availability of a heap space can be guaranteed as the statistics are provided via the V8 GetHeapSpaceStatistics function and may change from one V8 version to the next.

    The value returned is an array of objects containing the following properties:

    [
  {
    "space_name": "new_space",
    "space_size": 2063872,
    "space_used_size": 951112,
    "space_available_size": 80824,
    "physical_space_size": 2063872
  },
  {
    "space_name": "old_space",
    "space_size": 3090560,
    "space_used_size": 2493792,
    "space_available_size": 0,
    "physical_space_size": 3090560
  },
  {
    "space_name": "code_space",
    "space_size": 1260160,
    "space_used_size": 644256,
    "space_available_size": 960,
    "physical_space_size": 1260160
  },
  {
    "space_name": "map_space",
    "space_size": 1094160,
    "space_used_size": 201608,
    "space_available_size": 0,
    "physical_space_size": 1094160
  },
  {
    "space_name": "large_object_space",
    "space_size": 0,
    "space_used_size": 0,
    "space_available_size": 1490980608,
    "physical_space_size": 0
  }
]

  • Returns an object with the following properties:

    does_zap_garbage is a 0/1 boolean, which signifies whether the --zap_code_space option is enabled or not. This makes V8 overwrite heap garbage with a bit pattern. The RSS footprint (resident set size) gets bigger because it continuously touches all heap pages and that makes them less likely to get swapped out by the operating system.

    number_of_native_contexts The value of native_context is the number of the top-level contexts currently active. Increase of this number over time indicates a memory leak.

    number_of_detached_contexts The value of detached_context is the number of contexts that were detached and not yet garbage collected. This number being non-zero indicates a potential memory leak.

    total_global_handles_size The value of total_global_handles_size is the total memory size of V8 global handles.

    used_global_handles_size The value of used_global_handles_size is the used memory size of V8 global handles.

    external_memory The value of external_memory is the memory size of array buffers and external strings.

    {
  total_heap_size: 7326976,
  total_heap_size_executable: 4194304,
  total_physical_size: 7326976,
  total_available_size: 1152656,
  used_heap_size: 3476208,
  heap_size_limit: 1535115264,
  malloced_memory: 16384,
  peak_malloced_memory: 1127496,
  does_zap_garbage: 0,
  number_of_native_contexts: 1,
  number_of_detached_contexts: 0,
  total_global_handles_size: 8192,
  used_global_handles_size: 3296,
  external_memory: 318824
}
  • function queryObjects(
    ctor: Function
    ): number | string[];

    This is similar to the queryObjects() console API provided by the Chromium DevTools console. It can be used to search for objects that have the matching constructor on its prototype chain in the heap after a full garbage collection, which can be useful for memory leak regression tests. To avoid surprising results, users should avoid using this API on constructors whose implementation they don't control, or on constructors that can be invoked by other parties in the application.

    To avoid accidental leaks, this API does not return raw references to the objects found. By default, it returns the count of the objects found. If options.format is 'summary', it returns an array containing brief string representations for each object. The visibility provided in this API is similar to what the heap snapshot provides, while users can save the cost of serialization and parsing and directly filter the target objects during the search.

    Only objects created in the current execution context are included in the results.

    import { queryObjects } from 'node:v8';
class A { foo = 'bar'; }
console.log(queryObjects(A)); // 0
const a = new A();
console.log(queryObjects(A)); // 1
// [ "A { foo: 'bar' }" ]
console.log(queryObjects(A, { format: 'summary' }));

class B extends A { bar = 'qux'; }
const b = new B();
console.log(queryObjects(B)); // 1
// [ "B { foo: 'bar', bar: 'qux' }" ]
console.log(queryObjects(B, { format: 'summary' }));

// Note that, when there are child classes inheriting from a constructor,
// the constructor also shows up in the prototype chain of the child
// classes's prototoype, so the child classes's prototoype would also be
// included in the result.
console.log(queryObjects(A));  // 3
// [ "B { foo: 'bar', bar: 'qux' }", 'A {}', "A { foo: 'bar' }" ]
console.log(queryObjects(A, { format: 'summary' }));
    @param ctor

    The constructor that can be used to search on the prototype chain in order to filter target objects in the heap.

    function queryObjects(
    ctor: Function,
    options: { format: 'count' }
    ): number;

    This is similar to the queryObjects() console API provided by the Chromium DevTools console. It can be used to search for objects that have the matching constructor on its prototype chain in the heap after a full garbage collection, which can be useful for memory leak regression tests. To avoid surprising results, users should avoid using this API on constructors whose implementation they don't control, or on constructors that can be invoked by other parties in the application.

    To avoid accidental leaks, this API does not return raw references to the objects found. By default, it returns the count of the objects found. If options.format is 'summary', it returns an array containing brief string representations for each object. The visibility provided in this API is similar to what the heap snapshot provides, while users can save the cost of serialization and parsing and directly filter the target objects during the search.

    Only objects created in the current execution context are included in the results.

    import { queryObjects } from 'node:v8';
class A { foo = 'bar'; }
console.log(queryObjects(A)); // 0
const a = new A();
console.log(queryObjects(A)); // 1
// [ "A { foo: 'bar' }" ]
console.log(queryObjects(A, { format: 'summary' }));

class B extends A { bar = 'qux'; }
const b = new B();
console.log(queryObjects(B)); // 1
// [ "B { foo: 'bar', bar: 'qux' }" ]
console.log(queryObjects(B, { format: 'summary' }));

// Note that, when there are child classes inheriting from a constructor,
// the constructor also shows up in the prototype chain of the child
// classes's prototoype, so the child classes's prototoype would also be
// included in the result.
console.log(queryObjects(A));  // 3
// [ "B { foo: 'bar', bar: 'qux' }", 'A {}', "A { foo: 'bar' }" ]
console.log(queryObjects(A, { format: 'summary' }));
    @param ctor

    The constructor that can be used to search on the prototype chain in order to filter target objects in the heap.

    function queryObjects(
    ctor: Function,
    options: { format: 'summary' }
    ): string[];

    This is similar to the queryObjects() console API provided by the Chromium DevTools console. It can be used to search for objects that have the matching constructor on its prototype chain in the heap after a full garbage collection, which can be useful for memory leak regression tests. To avoid surprising results, users should avoid using this API on constructors whose implementation they don't control, or on constructors that can be invoked by other parties in the application.

    To avoid accidental leaks, this API does not return raw references to the objects found. By default, it returns the count of the objects found. If options.format is 'summary', it returns an array containing brief string representations for each object. The visibility provided in this API is similar to what the heap snapshot provides, while users can save the cost of serialization and parsing and directly filter the target objects during the search.

    Only objects created in the current execution context are included in the results.

    import { queryObjects } from 'node:v8';
class A { foo = 'bar'; }
console.log(queryObjects(A)); // 0
const a = new A();
console.log(queryObjects(A)); // 1
// [ "A { foo: 'bar' }" ]
console.log(queryObjects(A, { format: 'summary' }));

class B extends A { bar = 'qux'; }
const b = new B();
console.log(queryObjects(B)); // 1
// [ "B { foo: 'bar', bar: 'qux' }" ]
console.log(queryObjects(B, { format: 'summary' }));

// Note that, when there are child classes inheriting from a constructor,
// the constructor also shows up in the prototype chain of the child
// classes's prototoype, so the child classes's prototoype would also be
// included in the result.
console.log(queryObjects(A));  // 3
// [ "B { foo: 'bar', bar: 'qux' }", 'A {}', "A { foo: 'bar' }" ]
console.log(queryObjects(A, { format: 'summary' }));
    @param ctor

    The constructor that can be used to search on the prototype chain in order to filter target objects in the heap.

  • function serialize(
    value: any
    ): Buffer;

    Uses a DefaultSerializer to serialize value into a buffer.

    ERR_BUFFER_TOO_LARGE will be thrown when trying to serialize a huge object which requires buffer larger than buffer.constants.MAX_LENGTH.

  • flags: string
    ): void;

    The v8.setFlagsFromString() method can be used to programmatically set V8 command-line flags. This method should be used with care. Changing settings after the VM has started may result in unpredictable behavior, including crashes and data loss; or it may simply do nothing.

    The V8 options available for a version of Node.js may be determined by running node --v8-options.

    Usage:

    // Print GC events to stdout for one minute.
import v8 from 'node:v8';
v8.setFlagsFromString('--trace_gc');
setTimeout(() => { v8.setFlagsFromString('--notrace_gc'); }, 60e3);
  • limit: number
    ): void;

    The API is a no-op if --heapsnapshot-near-heap-limit is already set from the command line or the API is called more than once. limit must be a positive integer. See --heapsnapshot-near-heap-limit for more information.

  • function stopCoverage(): void;

    The v8.stopCoverage() method allows the user to stop the coverage collection started by NODE_V8_COVERAGE, so that V8 can release the execution count records and optimize code. This can be used in conjunction with takeCoverage if the user wants to collect the coverage on demand.

  • function takeCoverage(): void;

    The v8.takeCoverage() method allows the user to write the coverage started by NODE_V8_COVERAGE to disk on demand. This method can be invoked multiple times during the lifetime of the process. Each time the execution counter will be reset and a new coverage report will be written to the directory specified by NODE_V8_COVERAGE.

    When the process is about to exit, one last coverage will still be written to disk unless stopCoverage is invoked before the process exits.

  • filename?: string,
    options?: HeapSnapshotOptions
    ): string;

    Generates a snapshot of the current V8 heap and writes it to a JSON file. This file is intended to be used with tools such as Chrome DevTools. The JSON schema is undocumented and specific to the V8 engine, and may change from one version of V8 to the next.

    A heap snapshot is specific to a single V8 isolate. When using worker threads, a heap snapshot generated from the main thread will not contain any information about the workers, and vice versa.

    Creating a heap snapshot requires memory about twice the size of the heap at the time the snapshot is created. This results in the risk of OOM killers terminating the process.

    Generating a snapshot is a synchronous operation which blocks the event loop for a duration depending on the heap size.

    import { writeHeapSnapshot } from 'node:v8';
import {
  Worker,
  isMainThread,
  parentPort,
} from 'node:worker_threads';

if (isMainThread) {
  const worker = new Worker(__filename);

  worker.once('message', (filename) => {
    console.log(`worker heapdump: ${filename}`);
    // Now get a heapdump for the main thread.
    console.log(`main thread heapdump: ${writeHeapSnapshot()}`);
  });

  // Tell the worker to create a heapdump.
  worker.postMessage('heapdump');
} else {
  parentPort.once('message', (message) => {
    if (message === 'heapdump') {
      // Generate a heapdump for the worker
      // and return the filename to the parent.
      parentPort.postMessage(writeHeapSnapshot());
    }
  });
}
    @param filename

    The file path where the V8 heap snapshot is to be saved. If not specified, a file name with the pattern 'Heap-${yyyymmdd}-${hhmmss}-${pid}-${thread_id}.heapsnapshot' will be generated, where {pid} will be the PID of the Node.js process, {thread_id} will be 0 when writeHeapSnapshot() is called from the main Node.js thread or the id of a worker thread.

    @returns

    The filename where the snapshot was saved.

Type definitions