Source Code: lib/v8.js
The node: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('node:v8');
v8.cachedDataVersionTag()#
Added in: 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());
v8.setFlagsFromString('--allow_natives_syntax');
console.log(v8.cachedDataVersionTag());
v8.getHeapCodeStatistics()#
Added in: v12.8.0
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,
}
v8.getHeapSnapshot([options])#
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.
const v8 = require('node:v8');
const stream = v8.getHeapSnapshot();
stream.pipe(process.stdout);
v8.getHeapSpaceStatistics()#
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
}
]
v8.getHeapStatistics()#
Returns an object with the following properties:
total_heap_size The value of total_heap_size is the number of bytes V8 has
allocated for the heap. This can grow if used_heap needs more memory.
total_heap_size_executable The value of total_heap_size_executable is the
portion of the heap that can contain executable code, in bytes. This includes
memory used by JIT-compiled code and any memory that must be kept executable.
total_physical_size The value of total_physical_size is the actual physical memory
used by the V8 heap, in bytes. This is the amount of memory that is committed
(or in use) rather than reserved.
total_available_size The value of total_available_size is the number of
bytes of memory available to the V8 heap. This value represents how much
more memory V8 can use before it exceeds the heap limit.
used_heap_size The value of used_heap_size is number of bytes currently
being used by V8’s JavaScript objects. This is the actual memory in use and
does not include memory that has been allocated but not yet used.
heap_size_limit The value of heap_size_limit is the maximum size of the V8
heap, in bytes (either the default limit, determined by system resources, or
the value passed to the --max_old_space_size option).
malloced_memory The value of malloced_memory is the number of bytes allocated
through malloc by V8.
peak_malloced_memory The value of peak_malloced_memory is the peak number of
bytes allocated through malloc by V8 during the lifetime of the process.
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
}
v8.getCppHeapStatistics([detailLevel])#
Retrieves CppHeap statistics regarding memory consumption and
utilization using the V8 CollectStatistics() function which
may change from one V8 version to the
next.
detailLevel <string> | <undefined>: Default: '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.
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.
({
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',
});
({
committed_size_bytes: 131072,
resident_size_bytes: 131072,
used_size_bytes: 128864,
space_statistics: [],
type_names: [],
detail_level: 'brief',
});
v8.queryObjects(ctor[, options])#
Added in: v22.0.0
ctor <Function> The constructor that can be used to search on the
prototype chain in order to filter target objects in the heap.
options <undefined> | <Object>
format <string> If it's 'count', the count of matched objects
is returned. If it's 'summary', an array with summary strings
of the matched objects is returned.
- Returns: {number|Array}
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.
const { queryObjects } = require('node:v8');
class A { foo = 'bar'; }
console.log(queryObjects(A));
const a = new A();
console.log(queryObjects(A));
console.log(queryObjects(A, { format: 'summary' }));
class B extends A { bar = 'qux'; }
const b = new B();
console.log(queryObjects(B));
console.log(queryObjects(B, { format: 'summary' }));
console.log(queryObjects(A));
console.log(queryObjects(A, { format: 'summary' }));import { queryObjects } from 'node:v8';
class A { foo = 'bar'; }
console.log(queryObjects(A));
const a = new A();
console.log(queryObjects(A));
console.log(queryObjects(A, { format: 'summary' }));
class B extends A { bar = 'qux'; }
const b = new B();
console.log(queryObjects(B));
console.log(queryObjects(B, { format: 'summary' }));
console.log(queryObjects(A));
console.log(queryObjects(A, { format: 'summary' }));
v8.setFlagsFromString(flags)#
Added in: v1.0.0
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:
const v8 = require('node:v8');
v8.setFlagsFromString('--trace_gc');
setTimeout(() => { v8.setFlagsFromString('--notrace_gc'); }, 60e3);
v8.stopCoverage()#
Added in: v15.1.0, v14.18.0, v12.22.0
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
v8.takeCoverage() if the user wants to collect the coverage on demand.
v8.takeCoverage()#
Added in: 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 v8.stopCoverage() is invoked before the process exits.
v8.writeHeapSnapshot([filename[,options]])#
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.
options <Object>
exposeInternals <boolean> If true, expose internals in the heap snapshot.
Default: false.
exposeNumericValues <boolean> If true, expose numeric values in
artificial fields. Default: false.
- 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('node:v8');
const {
Worker,
isMainThread,
parentPort,
} = require('node:worker_threads');
if (isMainThread) {
const worker = new Worker(__filename);
worker.once('message', (filename) => {
console.log(`worker heapdump: ${filename}`);
console.log(`main thread heapdump: ${writeHeapSnapshot()}`);
});
worker.postMessage('heapdump');
} else {
parentPort.once('message', (message) => {
if (message === 'heapdump') {
parentPort.postMessage(writeHeapSnapshot());
}
});
}
v8.setHeapSnapshotNearHeapLimit(limit)#
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.
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.deserialize(buffer)#
Added in: v8.0.0
Uses a DefaultDeserializer with default options to read a JS value
from a buffer.
Class: v8.Serializer#
Added in: v8.0.0
new Serializer()#
Creates a new Serializer object.
serializer.writeHeader()
Writes out a header, which includes the serialization format version.
serializer.writeValue(value)#
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 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)#
Marks an ArrayBuffer as having its contents transferred out of band.
Pass the corresponding ArrayBuffer in the deserializing context to
deserializer.transferArrayBuffer().
serializer.writeUint64(hi, lo)#
Write a raw 64-bit unsigned integer, split into high and low 32-bit parts.
For use inside of a custom serializer._writeHostObject().
serializer.writeRawBytes(buffer)#
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)#
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)#
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)#
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)#
Indicate whether to treat TypedArray and DataView objects as
host objects, i.e. pass them to serializer._writeHostObject().
Class: v8.Deserializer#
Added in: v8.0.0
new Deserializer(buffer)#
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.getWireFormatVersion()#
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.readUint64()#
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._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#
Added in: 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.
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 'node:v8';
function init(promise, parent) {
console.log('a promise was created', { promise, parent });
}
function settled(promise) {
console.log('a promise resolved or rejected', { promise });
}
function before(promise) {
console.log('a promise is about to call a then handler', { promise });
}
function after(promise) {
console.log('a promise is done calling a then handler', { promise });
}
const stopWatchingInits = promiseHooks.onInit(init);
const stopWatchingSettleds = promiseHooks.onSettled(settled);
const stopWatchingBefores = promiseHooks.onBefore(before);
const stopWatchingAfters = promiseHooks.onAfter(after);
const stopHookSet = promiseHooks.createHook({
init,
settled,
before,
after,
});
stopWatchingInits();
stopWatchingSettleds();
stopWatchingBefores();
stopWatchingAfters();
stopHookSet();
promiseHooks.createHook(callbacks)#
Added in: v17.1.0, v16.14.0
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 'node:v8';
const stopAll = promiseHooks.createHook({
init(promise, parent) {},
});const { promiseHooks } = require('node: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)#
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)#
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)#
Called when the promise receives a resolution or rejection value. This may
occur synchronously in the case of Promise.resolve() or Promise.reject().
Startup Snapshot API#
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';
const fs = require('node:fs');
const zlib = require('node:zlib');
const path = require('node:path');
const assert = require('node:assert');
const v8 = require('node:v8');
class BookShelf {
storage = new Map();
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));
}
}
}
const shelf = new BookShelf(__dirname, [
'book1.en_US.txt',
'book1.es_ES.txt',
'book2.zh_CN.txt',
]);
assert(v8.startupSnapshot.isBuildingSnapshot());
v8.startupSnapshot.addSerializeCallback(BookShelf.compressAll, shelf);
v8.startupSnapshot.addDeserializeCallback(BookShelf.decompressAll, shelf);
v8.startupSnapshot.setDeserializeMainFunction((shelf) => {
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.
v8.startupSnapshot.addSerializeCallback(callback[, data])#
Added in: v18.6.0, v16.17.0
callback <Function> Callback to be invoked before serialization.
data <any> Optional data that will be passed to the callback when it
gets called.
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.
Callbacks are run in the order in which they are added.
v8.startupSnapshot.addDeserializeCallback(callback[, data])#
Added in: v18.6.0, v16.17.0
callback <Function> Callback to be invoked after the snapshot is
deserialized.
data <any> Optional data that will be passed to the callback when it
gets called.
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.
Callbacks are run in the order in which they are added.
v8.startupSnapshot.setDeserializeMainFunction(callback[, data])#
Added in: v18.6.0, v16.17.0
callback <Function> Callback to be invoked as the entry point after the
snapshot is deserialized.
data <any> Optional data that will be passed to the callback when it
gets called.
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.
v8.startupSnapshot.isBuildingSnapshot()#
Added in: v18.6.0, v16.17.0
Returns true if the Node.js instance is run to build a snapshot.
Class: v8.GCProfiler#
Added in: v19.6.0, v18.15.0
This API collects GC data in current thread.
new v8.GCProfiler()#
Added in: v19.6.0, v18.15.0
Create a new instance of the v8.GCProfiler class.
profiler.start()#
Added in: v19.6.0, v18.15.0
Start collecting GC data.
profiler.stop()#
Added in: v19.6.0, v18.15.0
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.
const { GCProfiler } = require('node:v8');
const profiler = new GCProfiler();
profiler.start();
setTimeout(() => {
console.log(profiler.stop());
}, 1000);
v8.isStringOneByteRepresentation(content)#
Added in: v22.15.0
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');