Node.js v17.9.0 documentation

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.

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
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.

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.

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.

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

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.

Promise hooks#

The promiseHooks interface can be used to track promise lifecycle events. To track all async activity, see async_hooks which internally uses this module to produce promise lifecycle events in addition to events for other async resources. For request context management, see AsyncLocalStorage.

import { promiseHooks } from 'v8';

// There are four lifecycle events produced by promises:

// The `init` event represents the creation of a promise. This could be a
// direct creation such as with `new Promise(...)` or a continuation such
// as `then()` or `catch()`. It also happens whenever an async function is
// called or does an `await`. If a continuation promise is created, the
// `parent` will be the promise it is a continuation from.
function init(promise, parent) {
  console.log('a promise was created', { promise, parent });
}

// The `settled` event happens when a promise receives a resolution or
// rejection value. This may happen synchronously such as when using
// `Promise.resolve()` on non-promise input.
function settled(promise) {
  console.log('a promise resolved or rejected', { promise });
}

// The `before` event runs immediately before a `then()` or `catch()` handler
// runs or an `await` resumes execution.
function before(promise) {
  console.log('a promise is about to call a then handler', { promise });
}

// The `after` event runs immediately after a `then()` handler runs or when
// an `await` begins after resuming from another.
function after(promise) {
  console.log('a promise is done calling a then handler', { promise });
}

// Lifecycle hooks may be started and stopped individually
const stopWatchingInits = promiseHooks.onInit(init);
const stopWatchingSettleds = promiseHooks.onSettled(settled);
const stopWatchingBefores = promiseHooks.onBefore(before);
const stopWatchingAfters = promiseHooks.onAfter(after);

// Or they may be started and stopped in groups
const stopHookSet = promiseHooks.createHook({
  init,
  settled,
  before,
  after
});

// To stop a hook, call the function returned at its creation.
stopWatchingInits();
stopWatchingSettleds();
stopWatchingBefores();
stopWatchingAfters();
stopHookSet();

promiseHooks.onInit(init)#

• init <Function> The init callback to call when a promise is created.
• Returns: <Function> Call to stop the hook.

The init hook must be a plain function. Providing an async function will throw as it would produce an infinite microtask loop.

import { promiseHooks } from 'v8';

const stop = promiseHooks.onInit((promise, parent) => {});const { promiseHooks } = require('v8');

const stop = promiseHooks.onInit((promise, parent) => {});

promiseHooks.onSettled(settled)#

• settled <Function> The settled callback to call when a promise is resolved or rejected.
• Returns: <Function> Call to stop the hook.

The settled hook must be a plain function. Providing an async function will throw as it would produce an infinite microtask loop.

import { promiseHooks } from 'v8';

const stop = promiseHooks.onSettled((promise) => {});const { promiseHooks } = require('v8');

const stop = promiseHooks.onSettled((promise) => {});

promiseHooks.onBefore(before)#

• before <Function> The before callback to call before a promise continuation executes.
• Returns: <Function> Call to stop the hook.

The before hook must be a plain function. Providing an async function will throw as it would produce an infinite microtask loop.

import { promiseHooks } from 'v8';

const stop = promiseHooks.onBefore((promise) => {});const { promiseHooks } = require('v8');

const stop = promiseHooks.onBefore((promise) => {});

promiseHooks.onAfter(after)#

• after <Function> The after callback to call after a promise continuation executes.
• Returns: <Function> Call to stop the hook.

The after hook must be a plain function. Providing an async function will throw as it would produce an infinite microtask loop.

import { promiseHooks } from 'v8';

const stop = promiseHooks.onAfter((promise) => {});const { promiseHooks } = require('v8');

const stop = promiseHooks.onAfter((promise) => {});

promiseHooks.createHook(callbacks)#

• callbacks <Object> The Hook Callbacks to register
  • init <Function> The init callback.
  • before <Function> The before callback.
  • after <Function> The after callback.
  • settled <Function> The settled callback.
• Returns: <Function> Used for disabling hooks

The hook callbacks must be plain functions. Providing async functions will throw as it would produce an infinite microtask loop.

Registers functions to be called for different lifetime events of each promise.

The callbacks init()/before()/after()/settled() are called for the respective events during a promise's lifetime.

All callbacks are optional. For example, if only promise creation needs to be tracked, then only the init callback needs to be passed. The specifics of all functions that can be passed to callbacks is in the Hook Callbacks section.

import { promiseHooks } from 'v8';

const stopAll = promiseHooks.createHook({
  init(promise, parent) {}
});const { promiseHooks } = require('v8');

const stopAll = promiseHooks.createHook({
  init(promise, parent) {}
});

Hook callbacks#

Key events in the lifetime of a promise have been categorized into four areas: creation of a promise, before/after a continuation handler is called or around an await, and when the promise resolves or rejects.

While these hooks are similar to those of async_hooks they lack a destroy hook. Other types of async resources typically represent sockets or file descriptors which have a distinct "closed" state to express the destroy lifecycle event while promises remain usable for as long as code can still reach them. Garbage collection tracking is used to make promises fit into the async_hooks event model, however this tracking is very expensive and they may not necessarily ever even be garbage collected.

Because promises are asynchronous resources whose lifecycle is tracked via the promise hooks mechanism, the init(), before(), after(), and settled() callbacks must not be async functions as they create more promises which would produce an infinite loop.

While this API is used to feed promise events into async_hooks, the ordering between the two is undefined. Both APIs are multi-tenant and therefore could produce events in any order relative to each other.

init(promise, parent)#

• promise <Promise> The promise being created.
• parent <Promise> The promise continued from, if applicable.

Called when a promise is constructed. This does not mean that corresponding before/after events will occur, only that the possibility exists. This will happen if a promise is created without ever getting a continuation.

before(promise)#

• promise <Promise>

Called before a promise continuation executes. This can be in the form of then(), catch(), or finally() handlers or an await resuming.

The before callback will be called 0 to N times. The before callback will typically be called 0 times if no continuation was ever made for the promise. The before callback may be called many times in the case where many continuations have been made from the same promise.

after(promise)#

• promise <Promise>

Called immediately after a promise continuation executes. This may be after a then(), catch(), or finally() handler or before an await after another await.

settled(promise)#

• promise <Promise>

Called when the promise receives a resolution or rejection value. This may occur synchronously in the case of Promise.resolve() or Promise.reject().