node/doc/api/v8.md

V8

The v8 module exposes APIs that are specific to the version of V8 built into the Node.js binary. It can be accessed using:

const v8 = require('v8');

v8.cachedDataVersionTag()

• Returns: {integer}

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

v8.getHeapCodeStatistics()

• Returns: {Object}

Returns an object with the following properties:

• code_and_metadata_size {number}
• bytecode_and_metadata_size {number}
• external_script_source_size {number}

{
  code_and_metadata_size: 212208,
  bytecode_and_metadata_size: 161368,
  external_script_source_size: 1410794
}

v8.getHeapSnapshot()

• Returns: {stream.Readable} A Readable Stream containing the V8 heap snapshot

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.

// Print heap snapshot to the console
const v8 = require('v8');
const stream = v8.getHeapSnapshot();
stream.pipe(process.stdout);

v8.getHeapSpaceStatistics()

• Returns: {Object[]}

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 {string}
• space_size {number}
• space_used_size {number}
• space_available_size {number}
• physical_space_size {number}

[
  {
    "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
  }
]

v8.getHeapStatistics()

• Returns: {Object}

Returns an object with the following properties:

• total_heap_size {number}
• total_heap_size_executable {number}
• total_physical_size {number}
• total_available_size {number}
• used_heap_size {number}
• heap_size_limit {number}
• malloced_memory {number}
• peak_malloced_memory {number}
• does_zap_garbage {number}
• number_of_native_contexts {number}
• number_of_detached_contexts {number}

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_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
}

v8.setFlagsFromString(flags)

• flags {string}

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.
const v8 = require('v8');
v8.setFlagsFromString('--trace_gc');
setTimeout(() => { v8.setFlagsFromString('--notrace_gc'); }, 60e3);

v8.stopCoverage()

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 v8.takeCoverage() if the user wants to collect the coverage on demand.

v8.takeCoverage()

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 v8.stopCoverage() is invoked before the process exits.

v8.writeHeapSnapshot([filename])

• filename {string} 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: {string} The filename where the snapshot was saved.

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.

const { writeHeapSnapshot } = require('v8');
const {
  Worker,
  isMainThread,
  parentPort
} = require('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());
    }
  });
}

Serialization API

The serialization API provides means of serializing JavaScript values in a way that is compatible with the HTML structured clone algorithm.

The format is backward-compatible (i.e. safe to store to disk). Equal JavaScript values may result in different serialized output.

v8.serialize(value)

• value {any}
• Returns: {Buffer}

Uses a DefaultSerializer to serialize value into a buffer.

v8.deserialize(buffer)

• buffer {Buffer|TypedArray|DataView} A buffer returned by serialize().

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

Class: v8.Serializer

new Serializer()

Creates a new Serializer object.

serializer.writeHeader()

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

serializer.writeValue(value)

• value {any}

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

This throws an error if value cannot be serialized.

serializer.releaseBuffer()

• Returns: {Buffer}

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.

serializer.transferArrayBuffer(id, arrayBuffer)

• id {integer} A 32-bit unsigned integer.
• arrayBuffer {ArrayBuffer} An ArrayBuffer instance.

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

serializer.writeUint32(value)

• value {integer}

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

serializer.writeUint64(hi, lo)

• hi {integer}
• lo {integer}

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

serializer.writeDouble(value)

• value {number}

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

serializer.writeRawBytes(buffer)

• buffer {Buffer|TypedArray|DataView}

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().

serializer._writeHostObject(object)

• object {Object}

This method is called to write some kind of host object, i.e. an object created by native C++ bindings. If it is not possible to serialize object, a suitable exception should be thrown.

This method is not present on the Serializer class itself but can be provided by subclasses.

serializer._getDataCloneError(message)

• message {string}

This method is called to generate error objects that will be thrown when an object can not be cloned.

This method defaults to the Error constructor and can be overridden on subclasses.

serializer._getSharedArrayBufferId(sharedArrayBuffer)

• sharedArrayBuffer {SharedArrayBuffer}

This method is called when the serializer is going to serialize a SharedArrayBuffer object. It must return an unsigned 32-bit integer ID for the object, using the same ID if this SharedArrayBuffer has already been serialized. When deserializing, this ID will be passed to deserializer.transferArrayBuffer().

If the object cannot be serialized, an exception should be thrown.

This method is not present on the Serializer class itself but can be provided by subclasses.

serializer._setTreatArrayBufferViewsAsHostObjects(flag)

• flag {boolean} Default: false

Indicate whether to treat TypedArray and DataView objects as host objects, i.e. pass them to serializer._writeHostObject().

Class: v8.Deserializer

new Deserializer(buffer)

• buffer {Buffer|TypedArray|DataView} A buffer returned by serializer.releaseBuffer().

Creates a new Deserializer object.

deserializer.readHeader()

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.

deserializer.readValue()

Deserializes a JavaScript value from the buffer and returns it.

deserializer.transferArrayBuffer(id, arrayBuffer)

• id {integer} A 32-bit unsigned integer.
• arrayBuffer {ArrayBuffer|SharedArrayBuffer} An ArrayBuffer instance.

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).

deserializer.getWireFormatVersion()

• Returns: {integer}

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().

deserializer.readUint32()

• Returns: {integer}

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

deserializer.readUint64()

• Returns: {integer[]}

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().

deserializer.readDouble()

• Returns: {number}

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

deserializer.readRawBytes(length)

• length {integer}
• Returns: {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().

deserializer._readHostObject()

This method is called to read some kind of host object, i.e. an object that is created by native C++ bindings. If it is not possible to deserialize the data, a suitable exception should be thrown.

This method is not present on the Deserializer class itself but can be provided by subclasses.

Class: v8.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.

Class: v8.DefaultDeserializer

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