- Assertion testing
- Asynchronous context tracking
- Async hooks
- Buffer
- C++ addons
- C/C++ addons with Node-API
- C++ embedder API
- Child processes
- Cluster
- Command-line options
- Console
- Corepack
- Crypto
- Debugger
- Deprecated APIs
- Diagnostics Channel
- DNS
- Domain
- Errors
- Events
- File system
- Globals
- HTTP
- HTTP/2
- HTTPS
- Inspector
- Internationalization
- Modules: CommonJS modules
- Modules: ECMAScript modules
- Modules:
node:module
API - Modules: Packages
- Net
- OS
- Path
- Performance hooks
- Permissions
- Process
- Punycode
- Query strings
- Readline
- REPL
- Report
- Stream
- String decoder
- Test runner
- Timers
- TLS/SSL
- Trace events
- TTY
- UDP/datagram
- URL
- Utilities
- V8
- VM
- WASI
- Web Crypto API
- Web Streams API
- Worker threads
- Zlib
Node.js v18.14.2-rc.0 documentation
- Node.js v18.14.2-rc.0
-
►
Table of contents
- V8
v8.cachedDataVersionTag()
v8.getHeapCodeStatistics()
v8.getHeapSnapshot()
v8.getHeapSpaceStatistics()
v8.getHeapStatistics()
v8.setFlagsFromString(flags)
v8.stopCoverage()
v8.takeCoverage()
v8.writeHeapSnapshot([filename])
v8.setHeapSnapshotNearHeapLimit(limit)
- Serialization API
v8.serialize(value)
v8.deserialize(buffer)
- Class:
v8.Serializer
new Serializer()
serializer.writeHeader()
serializer.writeValue(value)
serializer.releaseBuffer()
serializer.transferArrayBuffer(id, arrayBuffer)
serializer.writeUint32(value)
serializer.writeUint64(hi, lo)
serializer.writeDouble(value)
serializer.writeRawBytes(buffer)
serializer._writeHostObject(object)
serializer._getDataCloneError(message)
serializer._getSharedArrayBufferId(sharedArrayBuffer)
serializer._setTreatArrayBufferViewsAsHostObjects(flag)
- Class:
v8.Deserializer
new Deserializer(buffer)
deserializer.readHeader()
deserializer.readValue()
deserializer.transferArrayBuffer(id, arrayBuffer)
deserializer.getWireFormatVersion()
deserializer.readUint32()
deserializer.readUint64()
deserializer.readDouble()
deserializer.readRawBytes(length)
deserializer._readHostObject()
- Class:
v8.DefaultSerializer
- Class:
v8.DefaultDeserializer
- Promise hooks
- Startup Snapshot API
- V8
-
►
Index
- Assertion testing
- Asynchronous context tracking
- Async hooks
- Buffer
- C++ addons
- C/C++ addons with Node-API
- C++ embedder API
- Child processes
- Cluster
- Command-line options
- Console
- Corepack
- Crypto
- Debugger
- Deprecated APIs
- Diagnostics Channel
- DNS
- Domain
- Errors
- Events
- File system
- Globals
- HTTP
- HTTP/2
- HTTPS
- Inspector
- Internationalization
- Modules: CommonJS modules
- Modules: ECMAScript modules
- Modules:
node:module
API - Modules: Packages
- Net
- OS
- Path
- Performance hooks
- Permissions
- Process
- Punycode
- Query strings
- Readline
- REPL
- Report
- Stream
- String decoder
- Test runner
- Timers
- TLS/SSL
- Trace events
- TTY
- UDP/datagram
- URL
- Utilities
- V8
- VM
- WASI
- Web Crypto API
- Web Streams API
- Worker threads
- Zlib
- ► Other versions
- ► Options
Table of contents
- V8
v8.cachedDataVersionTag()
v8.getHeapCodeStatistics()
v8.getHeapSnapshot()
v8.getHeapSpaceStatistics()
v8.getHeapStatistics()
v8.setFlagsFromString(flags)
v8.stopCoverage()
v8.takeCoverage()
v8.writeHeapSnapshot([filename])
v8.setHeapSnapshotNearHeapLimit(limit)
- Serialization API
v8.serialize(value)
v8.deserialize(buffer)
- Class:
v8.Serializer
new Serializer()
serializer.writeHeader()
serializer.writeValue(value)
serializer.releaseBuffer()
serializer.transferArrayBuffer(id, arrayBuffer)
serializer.writeUint32(value)
serializer.writeUint64(hi, lo)
serializer.writeDouble(value)
serializer.writeRawBytes(buffer)
serializer._writeHostObject(object)
serializer._getDataCloneError(message)
serializer._getSharedArrayBufferId(sharedArrayBuffer)
serializer._setTreatArrayBufferViewsAsHostObjects(flag)
- Class:
v8.Deserializer
new Deserializer(buffer)
deserializer.readHeader()
deserializer.readValue()
deserializer.transferArrayBuffer(id, arrayBuffer)
deserializer.getWireFormatVersion()
deserializer.readUint32()
deserializer.readUint64()
deserializer.readDouble()
deserializer.readRawBytes(length)
deserializer._readHostObject()
- Class:
v8.DefaultSerializer
- Class:
v8.DefaultDeserializer
- Promise hooks
- Startup Snapshot API
V8#
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()
#
- 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>
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
<number>bytecode_and_metadata_size
<number>external_script_source_size
<number>cpu_profiler_metadata_size
<number>
{
code_and_metadata_size: 212208,
bytecode_and_metadata_size: 161368,
external_script_source_size: 1410794,
cpu_profiler_metadata_size: 0,
}
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('node: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>total_global_handles_size
<number>used_global_handles_size
<number>external_memory
<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_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.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('node: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 be0
whenwriteHeapSnapshot()
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('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}`);
// 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());
}
});
}
v8.setHeapSnapshotNearHeapLimit(limit)
#
limit
<integer>
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.serialize(value)
#
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 byserialize()
.
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> AnArrayBuffer
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)
#
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 byserializer.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> AnArrayBuffer
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 SharedArrayBuffer
s).
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)
#
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 ArrayBuffer
s 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 'node: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> Theinit
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 'node:v8';
const stop = promiseHooks.onInit((promise, parent) => {});
const { promiseHooks } = require('node:v8');
const stop = promiseHooks.onInit((promise, parent) => {});
promiseHooks.onSettled(settled)
#
settled
<Function> Thesettled
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 'node:v8';
const stop = promiseHooks.onSettled((promise) => {});
const { promiseHooks } = require('node:v8');
const stop = promiseHooks.onSettled((promise) => {});
promiseHooks.onBefore(before)
#
before
<Function> Thebefore
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 'node:v8';
const stop = promiseHooks.onBefore((promise) => {});
const { promiseHooks } = require('node:v8');
const stop = promiseHooks.onBefore((promise) => {});
promiseHooks.onAfter(after)
#
after
<Function> Theafter
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 'node:v8';
const stop = promiseHooks.onAfter((promise) => {});
const { promiseHooks } = require('node:v8');
const stop = promiseHooks.onAfter((promise) => {});
promiseHooks.createHook(callbacks)
#
callbacks
<Object> The Hook Callbacks to registerinit
<Function> Theinit
callback.before
<Function> Thebefore
callback.after
<Function> Theafter
callback.settled
<Function> Thesettled
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 '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)
#
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()
.
Startup Snapshot API#
The v8.startupSnapshot
interface can be used to add serialization and
deserialization hooks for custom startup snapshots. Currently the startup
snapshots can only be built into the Node.js binary from source.
$ cd /path/to/node
$ ./configure --node-snapshot-main=entry.js
$ make node
# This binary contains the result of the execution of entry.js
$ out/Release/node
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 {
isBuildingSnapshot,
addSerializeCallback,
addDeserializeCallback,
setDeserializeMainFunction,
} = require('node:v8').startupSnapshot;
const filePath = path.resolve(__dirname, '../x1024.txt');
const storage = {};
assert(isBuildingSnapshot());
addSerializeCallback(({ filePath }) => {
storage[filePath] = zlib.gzipSync(fs.readFileSync(filePath));
}, { filePath });
addDeserializeCallback(({ filePath }) => {
storage[filePath] = zlib.gunzipSync(storage[filePath]);
}, { filePath });
setDeserializeMainFunction(({ filePath }) => {
console.log(storage[filePath].toString());
}, { filePath });
The resulted binary will simply print the data deserialized from the snapshot during start up:
$ out/Release/node
# Prints content of ./test/fixtures/x1024.txt
Currently the API is only available to a Node.js instance launched from the default snapshot, that is, the application deserialized from a user-land snapshot cannot use these APIs again.
v8.startupSnapshot.addSerializeCallback(callback[, data])
#
callback
<Function> Callback to be invoked before serialization.data
<any> Optional data that will be passed to thecallback
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.
v8.startupSnapshot.addDeserializeCallback(callback[, data])
#
callback
<Function> Callback to be invoked after the snapshot is deserialized.data
<any> Optional data that will be passed to thecallback
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.
v8.startupSnapshot.setDeserializeMainFunction(callback[, data])
#
callback
<Function> Callback to be invoked as the entry point after the snapshot is deserialized.data
<any> Optional data that will be passed to thecallback
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()
#
- Returns: <boolean>
Returns true if the Node.js instance is run to build a snapshot.