Exports

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

@since v17.1.0, v16.14.0

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.

@since v18.6.0, v16.17.0

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

@since v8.0.0

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.

@since v8.0.0

@since v8.0.0

This API collects GC data in current thread.

@since v19.6.0, v18.15.0

@since v8.0.0

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

@since v8.0.0

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

@since v8.0.0

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',
});

@since v22.15.0

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,
}

@since v12.8.0

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);

@since v11.13.0

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
}
]

@since v6.0.0

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
}

@since v1.0.0

V8 only supports Latin-1/ISO-8859-1 and UTF16 as the underlying representation of a string. If the content uses Latin-1/ISO-8859-1 as the underlying representation, this function will return true; otherwise, it returns false.

If this method returns false, that does not mean that the string contains some characters not in Latin-1/ISO-8859-1. Sometimes a Latin-1 string may also be represented as UTF16.

const { isStringOneByteRepresentation } = require('node:v8');

const Encoding = {
latin1: 1,
utf16le: 2,
};
const buffer = Buffer.alloc(100);
function writeString(input) {
if (isStringOneByteRepresentation(input)) {
buffer.writeUint8(Encoding.latin1);
buffer.writeUint32LE(input.length, 1);
buffer.write(input, 5, 'latin1');
} else {
buffer.writeUint8(Encoding.utf16le);
buffer.writeUint32LE(input.length * 2, 1);
buffer.write(input, 5, 'utf16le');
}
}
writeString('hello');
writeString('你好');

@since v22.15.0

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' }));

@since v20.13.0

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' }));

@since v20.13.0

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' }));

@since v20.13.0

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.

@since v8.0.0

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);

@since v1.0.0

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.

@since v18.10.0, v16.18.0

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.

@since v15.1.0, v14.18.0, v12.22.0

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.

@since v15.1.0, v14.18.0, v12.22.0

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());
}
});
}

@since v11.13.0

Returns. The filename where the snapshot was saved.