v8

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

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

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,
}

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 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
}

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' }));

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.

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);

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.

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.

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.

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());
    }
  });
}