Node.js v8 module | API Reference | Bun

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BuildDocsReferenceGuidesBlogDiscord/node:v8FcachedDataVersionTagCDefaultDeserializerCDefaultSerializerFdeserializeCDeserializerCGCProfilerFgetCppHeapStatisticsFgetHeapCodeStatisticsFgetHeapSnapshotFgetHeapSpaceStatisticsFgetHeapStatisticsFisStringOneByteRepresentationVpromiseHooksFqueryObjectsFserializeCSerializerFsetFlagsFromStringFsetHeapSnapshotNearHeapLimitFstartCPUProfileNstartupSnapshotFstopCoverageFtakeCoverageFwriteHeapSnapshot

Node.js module

v8The 'node:v8' module exposes APIs for heap snapshots and memory profiling. In Node.js these are V8-specific; Bun provides compatible implementations using JavaScriptCore.

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`.

{ // 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: ```bash $ 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." data-algolia-static="false" data-algolia-merged="false" data-type="Namespace">namespace startupSnapshotThe 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 snapshotnode --snapshot-blob snapshot.blobIn 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 addDeserializeCallback(callback: StartupSnapshotCallbackFn,data?: any): void;Add a callback that will be called when the Node.js instance is deserialized from a snapshot. The callback and the data (if provided) will be serialized into the snapshot, they can be used to re-initialize the state of the application or to re-acquire resources that the application needs when the application is restarted from the snapshot.

function addSerializeCallback(callback: StartupSnapshotCallbackFn,data?: any): void;Add a callback that will be called when the Node.js instance is about to get serialized into a snapshot and exit. This can be used to release resources that should not or cannot be serialized or to convert user data into a form more suitable for serialization.

function isBuildingSnapshot(): boolean;Returns true if the Node.js instance is run to build a snapshot.

function setDeserializeMainFunction(callback: StartupSnapshotCallbackFn,data?: any): void;This sets the entry point of the Node.js application when it is deserialized from a snapshot. This can be called only once in the snapshot building script. If called, the deserialized application no longer needs an additional entry point script to start up and will simply invoke the callback along with the deserialized data (if provided), otherwise an entry point script still needs to be provided to the deserialized application.

class DefaultDeserializerA subclass of Deserializer corresponding to the format written by DefaultSerializer.

getWireFormatVersion(): number;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.

readRawBytes(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.

transferArrayBuffer(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 idA 32-bit unsigned integer.

@param arrayBufferAn ArrayBuffer instance.

class DefaultSerializerA 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.

releaseBuffer(): NonSharedBuffer;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.

transferArrayBuffer(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 idA 32-bit unsigned integer.

@param arrayBufferAn ArrayBuffer instance.

writeDouble(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.

writeRawBytes(buffer: ArrayBufferView): 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().

writeUint32(value: number): void;Write a raw 32-bit unsigned integer. For use inside of a custom serializer._writeHostObject().

writeUint64(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().

writeValue(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.

class DeserializergetWireFormatVersion(): number;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.

readRawBytes(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.

transferArrayBuffer(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 idA 32-bit unsigned integer.

@param arrayBufferAn ArrayBuffer instance.

class GCProfilerThis API collects GC data in current thread.

start(): void;Start collecting GC data.

{ console.log(profiler.stop()); }, 1000); ```" data-algolia-static="false" data-algolia-merged="false" data-type="Method">stop(): GCProfilerResult;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); class SerializerreleaseBuffer(): NonSharedBuffer;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.

transferArrayBuffer(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 idA 32-bit unsigned integer.

@param arrayBufferAn ArrayBuffer instance.

writeDouble(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.

writeRawBytes(buffer: ArrayBufferView): 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().

writeUint32(value: number): void;Write a raw 32-bit unsigned integer. For use inside of a custom serializer._writeHostObject().

writeUint64(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().

writeValue(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.

const promiseHooks: PromiseHooksThe promiseHooks interface can be used to track promise lifecycle events.

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 bufferA buffer returned by serialize.

function getCppHeapStatistics(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 detailLevelDefault: '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.function getHeapCodeStatistics(): HeapCodeStatistics;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): Readable;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); @returnsA Readable containing the V8 heap snapshot.

function getHeapSpaceStatistics(): HeapSpaceInfo[];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 } ] function getHeapStatistics(): HeapInfo;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_allocated_bytes The value of total allocated bytes since the Isolate creation

{ 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 isStringOneByteRepresentation(content: string): boolean;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'); 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 ctorThe 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 ctorThe 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 ctorThe constructor that can be used to search on the prototype chain in order to filter target objects in the heap.

function serialize(value: any): NonSharedBuffer;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.setFlagsFromString('--notrace_gc'); }, 60e3); ```" data-algolia-static="false" data-algolia-merged="false" data-type="Function">function setFlagsFromString(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); function setHeapSnapshotNearHeapLimit(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 startCPUProfile(): SyncCPUProfileHandle;Starting a CPU profile then return a SyncCPUProfileHandle object. This API supports using syntax.

const handle = v8.startCpuProfile(); const profile = handle.stop(); console.log(profile); 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.

{ 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()); } }); } ```" data-algolia-static="false" data-algolia-merged="false" data-type="Function">function writeHeapSnapshot(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: ${writeHeapSnapshot()}`); }); // Tell the worker to create a heapdump. worker.postMessage('heapdump'); } else { parentPort.once('message', (message) => { if (message === 'heapdump') { // Generate a heapdump for the worker // and return the filename to the parent. parentPort.postMessage(writeHeapSnapshot()); } }); } @param filenameThe file path where the V8 heap snapshot is to be saved. If not specified, a file name with the pattern 'Heap-${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.

@returnsThe filename where the snapshot was saved.

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

interface BeforeCalled 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.

interface CPUProfileHandle[Symbol.asyncDispose](): Promisevoid>;Stopping collecting the profile and the profile will be discarded.

stop(): Promisestring>;Stopping collecting the profile, then return a Promise that fulfills with an error or the profile data.

interface GCProfilerResultendTime: numberstartTime: numberstatistics: { afterGC: { heapSpaceStatistics: HeapSpaceStatistics[]; heapStatistics: HeapStatistics }; beforeGC: { heapSpaceStatistics: HeapSpaceStatistics[]; heapStatistics: HeapStatistics }; cost: number; gcType: string }[]version: numberinterface HeapCodeStatisticsbytecode_and_metadata_size: numbercode_and_metadata_size: numberexternal_script_source_size: numberinterface HeapInfodoes_zap_garbage: DoesZapCodeSpaceFlagexternal_memory: numberheap_size_limit: numbermalloced_memory: numbernumber_of_detached_contexts: numbernumber_of_native_contexts: numberpeak_malloced_memory: numbertotal_allocated_bytes: numbertotal_available_size: numbertotal_global_handles_size: numbertotal_heap_size: numbertotal_heap_size_executable: numbertotal_physical_size: numberused_global_handles_size: numberused_heap_size: numberinterface HeapProfileHandle[Symbol.asyncDispose](): Promisevoid>;Stopping collecting the profile and the profile will be discarded.

stop(): Promisestring>;Stopping collecting the profile, then return a Promise that fulfills with an error or the profile data.

interface HeapSnapshotOptionsexposeInternals?: booleanIf true, expose internals in the heap snapshot.

exposeNumericValues?: booleanIf true, expose numeric values in artificial fields.

interface HeapSpaceInfophysical_space_size: numberspace_available_size: numberspace_name: stringspace_size: numberspace_used_size: numberinterface HeapSpaceStatisticsphysicalSpaceSize: numberspaceAvailableSize: numberspaceName: stringspaceSize: numberspaceUsedSize: numberinterface HeapStatisticsexternalMemory: numberheapSizeLimit: numbermallocedMemory: numberpeakMallocedMemory: numbertotalAvailableSize: numbertotalGlobalHandlesSize: numbertotalHeapSize: numbertotalHeapSizeExecutable: numbertotalPhysicalSize: numberusedGlobalHandlesSize: numberusedHeapSize: numberinterface HookCallbacksKey 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.

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.

after?: Afterbefore?: Beforeinit?: Initsettled?: Settledinterface InitCalled 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.

interface PromiseHookscreateHook: (callbacks: HookCallbacks) => FunctionRegisters 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 hook callbacks must be plain functions. Providing async functions will throw as it would produce an infinite microtask loop.

onAfter: (after: After) => FunctionThe after hook must be a plain function. Providing an async function will throw as it would produce an infinite microtask loop.

onBefore: (before: Before) => FunctionThe before hook must be a plain function. Providing an async function will throw as it would produce an infinite microtask loop.

onInit: (init: Init) => FunctionThe init hook must be a plain function. Providing an async function will throw as it would produce an infinite microtask loop.

onSettled: (settled: Settled) => FunctionThe settled hook must be a plain function. Providing an async function will throw as it would produce an infinite microtask loop.

interface SettledCalled when the promise receives a resolution or rejection value. This may occur synchronously in the case of () or ().

interface SyncCPUProfileHandle[Symbol.dispose](): void;Stopping collecting the profile and the profile will be discarded.

stop(): string;Stopping collecting the profile and return the profile data.

type DoesZapCodeSpaceFlag = 0 | 1type StartupSnapshotCallbackFn = (args: any) => any

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