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Node.js v6.13.0-rc.1 Documentation
Table of Contents
- Process
- Process Events
- process.abort()
- process.arch
- process.argv
- process.argv0
- process.chdir(directory)
- process.config
- process.connected
- process.cpuUsage([previousValue])
- process.cwd()
- process.disconnect()
- process.env
- process.emitWarning(warning[, name][, ctor])
- process.execArgv
- process.execPath
- process.exit([code])
- process.exitCode
- process.getegid()
- process.geteuid()
- process.getgid()
- process.getgroups()
- process.getuid()
- process.hrtime([time])
- process.initgroups(user, extra_group)
- process.kill(pid[, signal])
- process.mainModule
- process.memoryUsage()
- process.nextTick(callback[, ...args])
- process.pid
- process.platform
- process.ppid
- process.release
- process.send(message[, sendHandle[, options]][, callback])
- process.setegid(id)
- process.seteuid(id)
- process.setgid(id)
- process.setgroups(groups)
- process.setuid(id)
- process.stderr
- process.stdin
- process.stdout
- process.title
- process.umask([mask])
- process.uptime()
- process.version
- process.versions
- Exit Codes
Process#
The process
object is a global
that provides information about, and control
over, the current Node.js process. As a global, it is always available to
Node.js applications without using require()
.
Process Events#
The process
object is an instance of EventEmitter
.
Event: 'beforeExit'#
The 'beforeExit'
event is emitted when Node.js empties its event loop and has
no additional work to schedule. Normally, the Node.js process will exit when
there is no work scheduled, but a listener registered on the 'beforeExit'
event can make asynchronous calls, and thereby cause the Node.js process to
continue.
The listener callback function is invoked with the value of
process.exitCode
passed as the only argument.
The 'beforeExit'
event is not emitted for conditions causing explicit
termination, such as calling process.exit()
or uncaught exceptions.
The 'beforeExit'
should not be used as an alternative to the 'exit'
event
unless the intention is to schedule additional work.
Event: 'disconnect'#
If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the 'disconnect'
event will be emitted when
the IPC channel is closed.
Event: 'exit'#
The 'exit'
event is emitted when the Node.js process is about to exit as a
result of either:
- The
process.exit()
method being called explicitly; - The Node.js event loop no longer having any additional work to perform.
There is no way to prevent the exiting of the event loop at this point, and once
all 'exit'
listeners have finished running the Node.js process will terminate.
The listener callback function is invoked with the exit code specified either
by the process.exitCode
property, or the exitCode
argument passed to the
process.exit()
method, as the only argument.
For example:
process.on('exit', (code) => {
console.log(`About to exit with code: ${code}`);
});
Listener functions must only perform synchronous operations. The Node.js
process will exit immediately after calling the 'exit'
event listeners
causing any additional work still queued in the event loop to be abandoned.
In the following example, for instance, the timeout will never occur:
process.on('exit', (code) => {
setTimeout(() => {
console.log('This will not run');
}, 0);
});
Event: 'message'#
If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the 'message'
event is emitted whenever a
message sent by a parent process using childprocess.send()
is received by
the child process.
The listener callback is invoked with the following arguments:
message
<Object> a parsed JSON object or primitive valuesendHandle
<Handle object> anet.Socket
ornet.Server
object, or undefined.
Event: 'rejectionHandled'#
The 'rejectionHandled'
event is emitted whenever a Promise
has been rejected
and an error handler was attached to it (using promise.catch()
, for
example) later than one turn of the Node.js event loop.
The listener callback is invoked with a reference to the rejected Promise
as
the only argument.
The Promise
object would have previously been emitted in an
'unhandledRejection'
event, but during the course of processing gained a
rejection handler.
There is no notion of a top level for a Promise
chain at which rejections can
always be handled. Being inherently asynchronous in nature, a Promise
rejection can be handled at a future point in time — possibly much later than
the event loop turn it takes for the 'unhandledRejection'
event to be emitted.
Another way of stating this is that, unlike in synchronous code where there is an ever-growing list of unhandled exceptions, with Promises there can be a growing-and-shrinking list of unhandled rejections.
In synchronous code, the 'uncaughtException'
event is emitted when the list of
unhandled exceptions grows.
In asynchronous code, the 'unhandledRejection'
event is emitted when the list
of unhandled rejections grows, and the 'rejectionHandled'
event is emitted
when the list of unhandled rejections shrinks.
For example:
const unhandledRejections = new Map();
process.on('unhandledRejection', (reason, p) => {
unhandledRejections.set(p, reason);
});
process.on('rejectionHandled', (p) => {
unhandledRejections.delete(p);
});
In this example, the unhandledRejections
Map
will grow and shrink over time,
reflecting rejections that start unhandled and then become handled. It is
possible to record such errors in an error log, either periodically (which is
likely best for long-running application) or upon process exit (which is likely
most convenient for scripts).
Event: 'uncaughtException'#
The 'uncaughtException'
event is emitted when an uncaught JavaScript
exception bubbles all the way back to the event loop. By default, Node.js
handles such exceptions by printing the stack trace to stderr
and exiting.
Adding a handler for the 'uncaughtException'
event overrides this default
behavior.
The listener function is called with the Error
object passed as the only
argument.
For example:
process.on('uncaughtException', (err) => {
fs.writeSync(1, `Caught exception: ${err}`);
});
setTimeout(() => {
console.log('This will still run.');
}, 500);
// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
console.log('This will not run.');
Warning: Using 'uncaughtException'
correctly#
Note that 'uncaughtException'
is a crude mechanism for exception handling
intended to be used only as a last resort. The event should not be used as
an equivalent to On Error Resume Next
. Unhandled exceptions inherently mean
that an application is in an undefined state. Attempting to resume application
code without properly recovering from the exception can cause additional
unforeseen and unpredictable issues.
Exceptions thrown from within the event handler will not be caught. Instead the process will exit with a non-zero exit code and the stack trace will be printed. This is to avoid infinite recursion.
Attempting to resume normally after an uncaught exception can be similar to pulling out of the power cord when upgrading a computer -- nine out of ten times nothing happens - but the 10th time, the system becomes corrupted.
The correct use of 'uncaughtException'
is to perform synchronous cleanup
of allocated resources (e.g. file descriptors, handles, etc) before shutting
down the process. It is not safe to resume normal operation after
'uncaughtException'
.
To restart a crashed application in a more reliable way, whether uncaughtException
is emitted or not, an external monitor should be employed in a separate process
to detect application failures and recover or restart as needed.
Event: 'unhandledRejection'#
The 'unhandledRejection
' event is emitted whenever a Promise
is rejected and
no error handler is attached to the promise within a turn of the event loop.
When programming with Promises, exceptions are encapsulated as "rejected
promises". Rejections can be caught and handled using promise.catch()
and
are propagated through a Promise
chain. The 'unhandledRejection'
event is
useful for detecting and keeping track of promises that were rejected whose
rejections have not yet been handled.
The listener function is called with the following arguments:
reason
<Error> | <any> The object with which the promise was rejected (typically anError
object).p
thePromise
that was rejected.
For example:
process.on('unhandledRejection', (reason, p) => {
console.log('Unhandled Rejection at: Promise', p, 'reason:', reason);
// application specific logging, throwing an error, or other logic here
});
somePromise.then((res) => {
return reportToUser(JSON.pasre(res)); // note the typo (`pasre`)
}); // no `.catch` or `.then`
The following will also trigger the 'unhandledRejection'
event to be
emitted:
function SomeResource() {
// Initially set the loaded status to a rejected promise
this.loaded = Promise.reject(new Error('Resource not yet loaded!'));
}
const resource = new SomeResource();
// no .catch or .then on resource.loaded for at least a turn
In this example case, it is possible to track the rejection as a developer error
as would typically be the case for other 'unhandledRejection'
events. To
address such failures, a non-operational
.catch(() => { })
handler may be attached to
resource.loaded
, which would prevent the 'unhandledRejection'
event from
being emitted. Alternatively, the 'rejectionHandled'
event may be used.
Event: 'warning'#
The 'warning'
event is emitted whenever Node.js emits a process warning.
A process warning is similar to an error in that it describes exceptional conditions that are being brought to the user's attention. However, warnings are not part of the normal Node.js and JavaScript error handling flow. Node.js can emit warnings whenever it detects bad coding practices that could lead to sub-optimal application performance, bugs or security vulnerabilities.
The listener function is called with a single warning
argument whose value is
an Error
object. There are three key properties that describe the warning:
name
<string> The name of the warning (currentlyWarning
by default).message
<string> A system-provided description of the warning.stack
<string> A stack trace to the location in the code where the warning was issued.
process.on('warning', (warning) => {
console.warn(warning.name); // Print the warning name
console.warn(warning.message); // Print the warning message
console.warn(warning.stack); // Print the stack trace
});
By default, Node.js will print process warnings to stderr
. The --no-warnings
command-line option can be used to suppress the default console output but the
'warning'
event will still be emitted by the process
object.
The following example illustrates the warning that is printed to stderr
when
too many listeners have been added to an event
$ node
> events.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> (node:38638) Warning: Possible EventEmitter memory leak detected. 2 foo
... listeners added. Use emitter.setMaxListeners() to increase limit
In contrast, the following example turns off the default warning output and
adds a custom handler to the 'warning'
event:
$ node --no-warnings
> var p = process.on('warning', (warning) => console.warn('Do not do that!'));
> events.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> Do not do that!
The --trace-warnings
command-line option can be used to have the default
console output for warnings include the full stack trace of the warning.
Emitting custom warnings#
The process.emitWarning()
method can be used to issue
custom or application specific warnings.
// Emit a warning using a string...
process.emitWarning('Something happened!');
// Prints: (node 12345) Warning: Something happened!
// Emit a warning using an object...
process.emitWarning('Something Happened!', 'CustomWarning');
// Prints: (node 12345) CustomWarning: Something happened!
// Emit a warning using a custom Error object...
class CustomWarning extends Error {
constructor(message) {
super(message);
this.name = 'CustomWarning';
Error.captureStackTrace(this, CustomWarning);
}
}
const myWarning = new CustomWarning('Something happened!');
process.emitWarning(myWarning);
// Prints: (node 12345) CustomWarning: Something happened!
Emitting custom deprecation warnings#
Custom deprecation warnings can be emitted by setting the name
of a custom
warning to DeprecationWarning
. For instance:
process.emitWarning('This API is deprecated', 'DeprecationWarning');
Or,
const err = new Error('This API is deprecated');
err.name = 'DeprecationWarning';
process.emitWarning(err);
Launching Node.js using the --throw-deprecation
command line flag will
cause custom deprecation warnings to be thrown as exceptions.
Using the --trace-deprecation
command line flag will cause the custom
deprecation to be printed to stderr
along with the stack trace.
Using the --no-deprecation
command line flag will suppress all reporting
of the custom deprecation.
The *-deprecation
command line flags only affect warnings that use the name
DeprecationWarning
.
Signal Events#
Signal events will be emitted when the Node.js process receives a signal. Please
refer to signal(7) for a listing of standard POSIX signal names such as
SIGINT
, SIGHUP
, etc.
The name of each event will be the uppercase common name for the signal (e.g.
'SIGINT'
for SIGINT
signals).
For example:
// Begin reading from stdin so the process does not exit.
process.stdin.resume();
process.on('SIGINT', () => {
console.log('Received SIGINT. Press Control-D to exit.');
});
Note: An easy way to send the SIGINT
signal is with <Ctrl>-C
in most
terminal programs.
It is important to take note of the following:
SIGUSR1
is reserved by Node.js to start the debugger. It's possible to install a listener but doing so will not stop the debugger from starting.SIGTERM
andSIGINT
have default handlers on non-Windows platforms that resets the terminal mode before exiting with code128 + signal number
. If one of these signals has a listener installed, its default behavior will be removed (Node.js will no longer exit).SIGPIPE
is ignored by default. It can have a listener installed.SIGHUP
is generated on Windows when the console window is closed, and on other platforms under various similar conditions, see signal(7). It can have a listener installed, however Node.js will be unconditionally terminated by Windows about 10 seconds later. On non-Windows platforms, the default behavior ofSIGHUP
is to terminate Node.js, but once a listener has been installed its default behavior will be removed.SIGTERM
is not supported on Windows, it can be listened on.SIGINT
from the terminal is supported on all platforms, and can usually be generated withCTRL+C
(though this may be configurable). It is not generated when terminal raw mode is enabled.SIGBREAK
is delivered on Windows when<Ctrl>+<Break>
is pressed, on non-Windows platforms it can be listened on, but there is no way to send or generate it.SIGWINCH
is delivered when the console has been resized. On Windows, this will only happen on write to the console when the cursor is being moved, or when a readable tty is used in raw mode.SIGKILL
cannot have a listener installed, it will unconditionally terminate Node.js on all platforms.SIGSTOP
cannot have a listener installed.SIGBUS
,SIGFPE
,SIGSEGV
andSIGILL
, when not raised artificially using kill(2), inherently leave the process in a state from which it is not safe to attempt to call JS listeners. Doing so might lead to the process hanging in an endless loop, since listeners attached usingprocess.on()
are called asynchronously and therefore unable to correct the underlying problem.
Note: Windows does not support sending signals, but Node.js offers some
emulation with process.kill()
, and subprocess.kill()
. Sending
signal 0
can be used to test for the existence of a process. Sending SIGINT
,
SIGTERM
, and SIGKILL
cause the unconditional termination of the target
process.
process.abort()#
The process.abort()
method causes the Node.js process to exit immediately and
generate a core file.
process.arch#
The process.arch
property returns a string identifying the operating system CPU
architecture for which the Node.js binary was compiled.
The current possible values are: 'arm'
, 'arm64'
, 'ia32'
, 'mips'
,
'mipsel'
, 'ppc'
, 'ppc64'
, 's390'
, 's390x'
, 'x32'
, and 'x64'
.
console.log(`This processor architecture is ${process.arch}`);
process.argv#
The process.argv
property returns an array containing the command line
arguments passed when the Node.js process was launched. The first element will
be process.execPath
. See process.argv0
if access to the original value of
argv[0]
is needed. The second element will be the path to the JavaScript
file being executed. The remaining elements will be any additional command line
arguments.
For example, assuming the following script for process-args.js
:
// print process.argv
process.argv.forEach((val, index) => {
console.log(`${index}: ${val}`);
});
Launching the Node.js process as:
$ node process-2.js one two=three four
Would generate the output:
0: /usr/local/bin/node
1: /Users/mjr/work/node/process-2.js
2: one
3: two=three
4: four
process.argv0#
The process.argv0
property stores a read-only copy of the original value of
argv[0]
passed when Node.js starts.
$ bash -c 'exec -a customArgv0 ./node'
> process.argv[0]
'/Volumes/code/external/node/out/Release/node'
> process.argv0
'customArgv0'
process.chdir(directory)#
directory
<string>
The process.chdir()
method changes the current working directory of the
Node.js process or throws an exception if doing so fails (for instance, if
the specified directory
does not exist).
console.log(`Starting directory: ${process.cwd()}`);
try {
process.chdir('/tmp');
console.log(`New directory: ${process.cwd()}`);
} catch (err) {
console.error(`chdir: ${err}`);
}
process.config#
The process.config
property returns an Object containing the JavaScript
representation of the configure options used to compile the current Node.js
executable. This is the same as the config.gypi
file that was produced when
running the ./configure
script.
An example of the possible output looks like:
{
target_defaults:
{ cflags: [],
default_configuration: 'Release',
defines: [],
include_dirs: [],
libraries: [] },
variables:
{
host_arch: 'x64',
node_install_npm: 'true',
node_prefix: '',
node_shared_cares: 'false',
node_shared_http_parser: 'false',
node_shared_libuv: 'false',
node_shared_zlib: 'false',
node_use_dtrace: 'false',
node_use_openssl: 'true',
node_shared_openssl: 'false',
strict_aliasing: 'true',
target_arch: 'x64',
v8_use_snapshot: 'true'
}
}
Note: The process.config
property is not read-only and there are
existing modules in the ecosystem that are known to extend, modify, or entirely
replace the value of process.config
.
process.connected#
If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the process.connected
property will return
true
so long as the IPC channel is connected and will return false
after
process.disconnect()
is called.
Once process.connected
is false
, it is no longer possible to send messages
over the IPC channel using process.send()
.
process.cpuUsage([previousValue])#
The process.cpuUsage()
method returns the user and system CPU time usage of
the current process, in an object with properties user
and system
, whose
values are microsecond values (millionth of a second). These values measure time
spent in user and system code respectively, and may end up being greater than
actual elapsed time if multiple CPU cores are performing work for this process.
The result of a previous call to process.cpuUsage()
can be passed as the
argument to the function, to get a diff reading.
const startUsage = process.cpuUsage();
// { user: 38579, system: 6986 }
// spin the CPU for 500 milliseconds
const now = Date.now();
while (Date.now() - now < 500);
console.log(process.cpuUsage(startUsage));
// { user: 514883, system: 11226 }
process.cwd()#
- Returns: <string>
The process.cwd()
method returns the current working directory of the Node.js
process.
console.log(`Current directory: ${process.cwd()}`);
process.disconnect()#
If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the process.disconnect()
method will close the
IPC channel to the parent process, allowing the child process to exit gracefully
once there are no other connections keeping it alive.
The effect of calling process.disconnect()
is that same as calling the parent
process's ChildProcess.disconnect()
.
If the Node.js process was not spawned with an IPC channel,
process.disconnect()
will be undefined
.
process.env#
The process.env
property returns an object containing the user environment.
See environ(7).
An example of this object looks like:
{
TERM: 'xterm-256color',
SHELL: '/usr/local/bin/bash',
USER: 'maciej',
PATH: '~/.bin/:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin',
PWD: '/Users/maciej',
EDITOR: 'vim',
SHLVL: '1',
HOME: '/Users/maciej',
LOGNAME: 'maciej',
_: '/usr/local/bin/node'
}
It is possible to modify this object, but such modifications will not be reflected outside the Node.js process. In other words, the following example would not work:
$ node -e 'process.env.foo = "bar"' && echo $foo
While the following will:
process.env.foo = 'bar';
console.log(process.env.foo);
Assigning a property on process.env
will implicitly convert the value
to a string.
Example:
process.env.test = null;
console.log(process.env.test);
// => 'null'
process.env.test = undefined;
console.log(process.env.test);
// => 'undefined'
Use delete
to delete a property from process.env
.
Example:
process.env.TEST = 1;
delete process.env.TEST;
console.log(process.env.TEST);
// => undefined
On Windows operating systems, environment variables are case-insensitive.
Example:
process.env.TEST = 1;
console.log(process.env.test);
// => 1
process.emitWarning(warning[, name][, ctor])#
warning
<string> | <Error> The warning to emit.name
<string> Whenwarning
is a String,name
is the name to use for the warning. Default:Warning
.ctor
<Function> Whenwarning
is a String,ctor
is an optional function used to limit the generated stack trace. Defaultprocess.emitWarning
The process.emitWarning()
method can be used to emit custom or application
specific process warnings. These can be listened for by adding a handler to the
process.on('warning')
event.
// Emit a warning using a string...
process.emitWarning('Something happened!');
// Emits: (node: 56338) Warning: Something happened!
// Emit a warning using a string and a name...
process.emitWarning('Something Happened!', 'CustomWarning');
// Emits: (node:56338) CustomWarning: Something Happened!
In each of the previous examples, an Error
object is generated internally by
process.emitWarning()
and passed through to the
process.on('warning')
event.
process.on('warning', (warning) => {
console.warn(warning.name);
console.warn(warning.message);
console.warn(warning.stack);
});
If warning
is passed as an Error
object, it will be passed through to the
process.on('warning')
event handler unmodified (and the optional name
and ctor
arguments will be ignored):
// Emit a warning using an Error object...
const myWarning = new Error('Warning! Something happened!');
myWarning.name = 'CustomWarning';
process.emitWarning(myWarning);
// Emits: (node:56338) CustomWarning: Warning! Something Happened!
A TypeError
is thrown if warning
is anything other than a string or Error
object.
Note that while process warnings use Error
objects, the process warning
mechanism is not a replacement for normal error handling mechanisms.
The following additional handling is implemented if the warning name
is
DeprecationWarning
:
- If the
--throw-deprecation
command-line flag is used, the deprecation warning is thrown as an exception rather than being emitted as an event. - If the
--no-deprecation
command-line flag is used, the deprecation warning is suppressed. - If the
--trace-deprecation
command-line flag is used, the deprecation warning is printed tostderr
along with the full stack trace.
Avoiding duplicate warnings#
As a best practice, warnings should be emitted only once per process. To do
so, it is recommended to place the emitWarning()
behind a simple boolean
flag as illustrated in the example below:
let warned = false;
function emitMyWarning() {
if (!warned) {
process.emitWarning('Only warn once!');
warned = true;
}
}
emitMyWarning();
// Emits: (node: 56339) Warning: Only warn once!
emitMyWarning();
// Emits nothing
process.execArgv#
The process.execArgv
property returns the set of Node.js-specific command-line
options passed when the Node.js process was launched. These options do not
appear in the array returned by the process.argv
property, and do not
include the Node.js executable, the name of the script, or any options following
the script name. These options are useful in order to spawn child processes with
the same execution environment as the parent.
For example:
$ node --harmony script.js --version
Results in process.execArgv
:
['--harmony']
And process.argv
:
['/usr/local/bin/node', 'script.js', '--version']
process.execPath#
The process.execPath
property returns the absolute pathname of the executable
that started the Node.js process.
For example:
'/usr/local/bin/node'
process.exit([code])#
code
<integer> The exit code. Defaults to0
.
The process.exit()
method instructs Node.js to terminate the process
synchronously with an exit status of code
. If code
is omitted, exit uses
either the 'success' code 0
or the value of process.exitCode
if it has been
set. Node.js will not terminate until all the 'exit'
event listeners are
called.
To exit with a 'failure' code:
process.exit(1);
The shell that executed Node.js should see the exit code as 1
.
It is important to note that calling process.exit()
will force the process to
exit as quickly as possible even if there are still asynchronous operations
pending that have not yet completed fully, including I/O operations to
process.stdout
and process.stderr
.
In most situations, it is not actually necessary to call process.exit()
explicitly. The Node.js process will exit on its own if there is no additional
work pending in the event loop. The process.exitCode
property can be set to
tell the process which exit code to use when the process exits gracefully.
For instance, the following example illustrates a misuse of the
process.exit()
method that could lead to data printed to stdout being
truncated and lost:
// This is an example of what *not* to do:
if (someConditionNotMet()) {
printUsageToStdout();
process.exit(1);
}
The reason this is problematic is because writes to process.stdout
in Node.js
are sometimes asynchronous and may occur over multiple ticks of the Node.js
event loop. Calling process.exit()
, however, forces the process to exit
before those additional writes to stdout
can be performed.
Rather than calling process.exit()
directly, the code should set the
process.exitCode
and allow the process to exit naturally by avoiding
scheduling any additional work for the event loop:
// How to properly set the exit code while letting
// the process exit gracefully.
if (someConditionNotMet()) {
printUsageToStdout();
process.exitCode = 1;
}
If it is necessary to terminate the Node.js process due to an error condition,
throwing an uncaught error and allowing the process to terminate accordingly
is safer than calling process.exit()
.
process.exitCode#
A number which will be the process exit code, when the process either
exits gracefully, or is exited via process.exit()
without specifying
a code.
Specifying a code to process.exit(code)
will override any
previous setting of process.exitCode
.
process.getegid()#
The process.getegid()
method returns the numerical effective group identity
of the Node.js process. (See getegid(2).)
if (process.getegid) {
console.log(`Current gid: ${process.getegid()}`);
}
Note: This function is only available on POSIX platforms (i.e. not Windows or Android)
process.geteuid()#
- Returns: <Object>
The process.geteuid()
method returns the numerical effective user identity of
the process. (See geteuid(2).)
if (process.geteuid) {
console.log(`Current uid: ${process.geteuid()}`);
}
Note: This function is only available on POSIX platforms (i.e. not Windows or Android)
process.getgid()#
- Returns: <Object>
The process.getgid()
method returns the numerical group identity of the
process. (See getgid(2).)
if (process.getgid) {
console.log(`Current gid: ${process.getgid()}`);
}
Note: This function is only available on POSIX platforms (i.e. not Windows or Android)
process.getgroups()#
- Returns: <Array>
The process.getgroups()
method returns an array with the supplementary group
IDs. POSIX leaves it unspecified if the effective group ID is included but
Node.js ensures it always is.
Note: This function is only available on POSIX platforms (i.e. not Windows or Android)
process.getuid()#
- Returns: <integer>
The process.getuid()
method returns the numeric user identity of the process.
(See getuid(2).)
if (process.getuid) {
console.log(`Current uid: ${process.getuid()}`);
}
Note: This function is only available on POSIX platforms (i.e. not Windows or Android)
process.hrtime([time])#
The process.hrtime()
method returns the current high-resolution real time in a
[seconds, nanoseconds]
tuple Array. time
is an optional parameter that must
be the result of a previous process.hrtime()
call (and therefore, a real time
in a [seconds, nanoseconds]
tuple Array containing a previous time) to diff
with the current time. These times are relative to an arbitrary time in the
past, and not related to the time of day and therefore not subject to clock
drift. The primary use is for measuring performance between intervals.
Passing in the result of a previous call to process.hrtime()
is useful for
calculating an amount of time passed between calls:
const time = process.hrtime();
// [ 1800216, 25 ]
setTimeout(() => {
const diff = process.hrtime(time);
// [ 1, 552 ]
console.log(`Benchmark took ${diff[0] * 1e9 + diff[1]} nanoseconds`);
// benchmark took 1000000527 nanoseconds
}, 1000);
Constructing an array by some method other than calling process.hrtime()
and
passing the result to process.hrtime() will result in undefined behavior.
process.initgroups(user, extra_group)#
user
<string> | <number> The user name or numeric identifier.extra_group
<string> | <number> A group name or numeric identifier.
The process.initgroups()
method reads the /etc/group
file and initializes
the group access list, using all groups of which the user is a member. This is
a privileged operation that requires that the Node.js process either have root
access or the CAP_SETGID
capability.
Note that care must be taken when dropping privileges. Example:
console.log(process.getgroups()); // [ 0 ]
process.initgroups('bnoordhuis', 1000); // switch user
console.log(process.getgroups()); // [ 27, 30, 46, 1000, 0 ]
process.setgid(1000); // drop root gid
console.log(process.getgroups()); // [ 27, 30, 46, 1000 ]
Note: This function is only available on POSIX platforms (i.e. not Windows or Android)
process.kill(pid[, signal])#
pid
<number> A process IDsignal
<string> | <number> The signal to send, either as a string or number. Defaults to'SIGTERM'
.
The process.kill()
method sends the signal
to the process identified by
pid
.
Signal names are strings such as 'SIGINT'
or 'SIGHUP'
. See Signal Events
and kill(2) for more information.
This method will throw an error if the target pid
does not exist. As a special
case, a signal of 0
can be used to test for the existence of a process.
Windows platforms will throw an error if the pid
is used to kill a process
group.
Note:Even though the name of this function is process.kill()
, it is really
just a signal sender, like the kill
system call. The signal sent may do
something other than kill the target process.
For example:
process.on('SIGHUP', () => {
console.log('Got SIGHUP signal.');
});
setTimeout(() => {
console.log('Exiting.');
process.exit(0);
}, 100);
process.kill(process.pid, 'SIGHUP');
Note: When SIGUSR1
is received by a Node.js process, Node.js will start the
debugger, see Signal Events.
process.mainModule#
The process.mainModule
property provides an alternative way of retrieving
require.main
. The difference is that if the main module changes at
runtime, require.main
may still refer to the original main module in
modules that were required before the change occurred. Generally, it's
safe to assume that the two refer to the same module.
As with require.main
, process.mainModule
will be undefined
if there
is no entry script.
process.memoryUsage()#
The process.memoryUsage()
method returns an object describing the memory usage
of the Node.js process measured in bytes.
For example, the code:
console.log(process.memoryUsage());
Will generate:
{
rss: 4935680,
heapTotal: 1826816,
heapUsed: 650472,
external: 49879
}
heapTotal
and heapUsed
refer to V8's memory usage.
external
refers to the memory usage of C++ objects bound to JavaScript
objects managed by V8. rss
, Resident Set Size, is the amount of space
occupied in the main memory device (that is a subset of the total allocated
memory) for the process, which includes the heap, code segment and stack.
The heap is where objects, strings and closures are stored. Variables are stored in the stack and the actual JavaScript code resides in the code segment.
process.nextTick(callback[, ...args])#
callback
<Function>...args
<any> Additional arguments to pass when invoking thecallback
The process.nextTick()
method adds the callback
to the "next tick queue".
Once the current turn of the event loop turn runs to completion, all callbacks
currently in the next tick queue will be called.
This is not a simple alias to setTimeout(fn, 0)
. It is much more
efficient. It runs before any additional I/O events (including
timers) fire in subsequent ticks of the event loop.
console.log('start');
process.nextTick(() => {
console.log('nextTick callback');
});
console.log('scheduled');
// Output:
// start
// scheduled
// nextTick callback
This is important when developing APIs in order to give users the opportunity to assign event handlers after an object has been constructed but before any I/O has occurred:
function MyThing(options) {
this.setupOptions(options);
process.nextTick(() => {
this.startDoingStuff();
});
}
const thing = new MyThing();
thing.getReadyForStuff();
// thing.startDoingStuff() gets called now, not before.
It is very important for APIs to be either 100% synchronous or 100% asynchronous. Consider this example:
// WARNING! DO NOT USE! BAD UNSAFE HAZARD!
function maybeSync(arg, cb) {
if (arg) {
cb();
return;
}
fs.stat('file', cb);
}
This API is hazardous because in the following case:
const maybeTrue = Math.random() > 0.5;
maybeSync(maybeTrue, () => {
foo();
});
bar();
It is not clear whether foo()
or bar()
will be called first.
The following approach is much better:
function definitelyAsync(arg, cb) {
if (arg) {
process.nextTick(cb);
return;
}
fs.stat('file', cb);
}
Note: the next tick queue is completely drained on each pass of the
event loop before additional I/O is processed. As a result,
recursively setting nextTick callbacks will block any I/O from
happening, just like a while(true);
loop.
process.pid#
The process.pid
property returns the PID of the process.
console.log(`This process is pid ${process.pid}`);
process.platform#
The process.platform
property returns a string identifying the operating
system platform on which the Node.js process is running. For instance
'darwin'
, 'freebsd'
, 'linux'
, 'sunos'
or 'win32'
console.log(`This platform is ${process.platform}`);
process.ppid#
The process.ppid
property returns the PID of the current parent process.
console.log(`The parent process is pid ${process.ppid}`);
process.release#
The process.release
property returns an Object containing metadata related to
the current release, including URLs for the source tarball and headers-only
tarball.
process.release
contains the following properties:
name
<string> A value that will always be'node'
for Node.js. For legacy io.js releases, this will be'io.js'
.lts
: a string with a value indicating the codename of the LTS (Long-term Support) line the current release is part of. This property only exists for LTS releases and isundefined
for all other release types, including stable releases. Current valid values are:"Argon"
for the v4.x LTS line beginning with v4.2.0."Boron"
for the v6.x LTS line beginning with v6.9.0.
sourceUrl
<string> an absolute URL pointing to a.tar.gz
file containing the source code of the current release.headersUrl
<string> an absolute URL pointing to a.tar.gz
file containing only the source header files for the current release. This file is significantly smaller than the full source file and can be used for compiling Node.js native add-ons.libUrl
<string> an absolute URL pointing to anode.lib
file matching the architecture and version of the current release. This file is used for compiling Node.js native add-ons. This property is only present on Windows builds of Node.js and will be missing on all other platforms.lts
<string> a string label identifying the LTS label for this release. If the Node.js release is not an LTS release, this will beundefined
.
For example:
{
name: 'node',
lts: 'Argon',
sourceUrl: 'https://nodejs.org/download/release/v4.4.5/node-v4.4.5.tar.gz',
headersUrl: 'https://nodejs.org/download/release/v4.4.5/node-v4.4.5-headers.tar.gz',
libUrl: 'https://nodejs.org/download/release/v4.4.5/win-x64/node.lib'
}
In custom builds from non-release versions of the source tree, only the
name
property may be present. The additional properties should not be
relied upon to exist.
process.send(message[, sendHandle[, options]][, callback])#
message
<Object>sendHandle
<Handle object>options
<Object>callback
<Function>- Returns: <boolean>
If Node.js is spawned with an IPC channel, the process.send()
method can be
used to send messages to the parent process. Messages will be received as a
'message'
event on the parent's ChildProcess
object.
If Node.js was not spawned with an IPC channel, process.send()
will be
undefined
.
Note: This function uses JSON.stringify()
internally to serialize the
message
.*
process.setegid(id)#
The process.setegid()
method sets the effective group identity of the process.
(See setegid(2).) The id
can be passed as either a numeric ID or a group
name string. If a group name is specified, this method blocks while resolving
the associated a numeric ID.
if (process.getegid && process.setegid) {
console.log(`Current gid: ${process.getegid()}`);
try {
process.setegid(501);
console.log(`New gid: ${process.getegid()}`);
} catch (err) {
console.log(`Failed to set gid: ${err}`);
}
}
Note: This function is only available on POSIX platforms (i.e. not Windows or Android)
process.seteuid(id)#
The process.seteuid()
method sets the effective user identity of the process.
(See seteuid(2).) The id
can be passed as either a numeric ID or a username
string. If a username is specified, the method blocks while resolving the
associated numeric ID.
if (process.geteuid && process.seteuid) {
console.log(`Current uid: ${process.geteuid()}`);
try {
process.seteuid(501);
console.log(`New uid: ${process.geteuid()}`);
} catch (err) {
console.log(`Failed to set uid: ${err}`);
}
}
Note: This function is only available on POSIX platforms (i.e. not Windows or Android)
process.setgid(id)#
The process.setgid()
method sets the group identity of the process. (See
setgid(2).) The id
can be passed as either a numeric ID or a group name
string. If a group name is specified, this method blocks while resolving the
associated numeric ID.
if (process.getgid && process.setgid) {
console.log(`Current gid: ${process.getgid()}`);
try {
process.setgid(501);
console.log(`New gid: ${process.getgid()}`);
} catch (err) {
console.log(`Failed to set gid: ${err}`);
}
}
Note: This function is only available on POSIX platforms (i.e. not Windows or Android)
process.setgroups(groups)#
groups
<Array>
The process.setgroups()
method sets the supplementary group IDs for the
Node.js process. This is a privileged operation that requires the Node.js process
to have root
or the CAP_SETGID
capability.
The groups
array can contain numeric group IDs, group names or both.
Note: This function is only available on POSIX platforms (i.e. not Windows or Android)
process.setuid(id)#
The process.setuid(id)
method sets the user identity of the process. (See
setuid(2).) The id
can be passed as either a numeric ID or a username string.
If a username is specified, the method blocks while resolving the associated
numeric ID.
if (process.getuid && process.setuid) {
console.log(`Current uid: ${process.getuid()}`);
try {
process.setuid(501);
console.log(`New uid: ${process.getuid()}`);
} catch (err) {
console.log(`Failed to set uid: ${err}`);
}
}
Note: This function is only available on POSIX platforms (i.e. not Windows or Android)
process.stderr#
The process.stderr
property returns a stream connected to
stderr
(fd 2
). It is a net.Socket
(which is a Duplex
stream) unless fd 2
refers to a file, in which case it is
a Writable stream.
Note: process.stderr
differs from other Node.js streams in important ways,
see note on process I/O for more information.
process.stdin#
The process.stdin
property returns a stream connected to
stdin
(fd 0
). It is a net.Socket
(which is a Duplex
stream) unless fd 0
refers to a file, in which case it is
a Readable stream.
For example:
process.stdin.setEncoding('utf8');
process.stdin.on('readable', () => {
const chunk = process.stdin.read();
if (chunk !== null) {
process.stdout.write(`data: ${chunk}`);
}
});
process.stdin.on('end', () => {
process.stdout.write('end');
});
As a Duplex stream, process.stdin
can also be used in "old" mode that
is compatible with scripts written for Node.js prior to v0.10.
For more information see Stream compatibility.
Note: In "old" streams mode the stdin
stream is paused by default, so one
must call process.stdin.resume()
to read from it. Note also that calling
process.stdin.resume()
itself would switch stream to "old" mode.
process.stdout#
The process.stdout
property returns a stream connected to
stdout
(fd 1
). It is a net.Socket
(which is a Duplex
stream) unless fd 1
refers to a file, in which case it is
a Writable stream.
For example, to copy process.stdin to process.stdout:
process.stdin.pipe(process.stdout);
Note: process.stdout
differs from other Node.js streams in important ways,
see note on process I/O for more information.
A note on process I/O#
process.stdout
and process.stderr
differ from other Node.js streams in
important ways:
- They are used internally by
console.log()
andconsole.error()
, respectively. - They cannot be closed (
end()
will throw). - They will never emit the
'finish'
event. - Writes may be synchronous depending on what the stream is connected to
and whether the system is Windows or POSIX:
- Files: synchronous on Windows and POSIX
- TTYs (Terminals): asynchronous on Windows, synchronous on POSIX
- Pipes (and sockets): synchronous on Windows, asynchronous on POSIX
These behaviors are partly for historical reasons, as changing them would create backwards incompatibility, but they are also expected by some users.
Synchronous writes avoid problems such as output written with console.log()
or
console.error()
being unexpectedly interleaved, or not written at all if
process.exit()
is called before an asynchronous write completes. See
process.exit()
for more information.
Warning: Synchronous writes block the event loop until the write has completed. This can be near instantaneous in the case of output to a file, but under high system load, pipes that are not being read at the receiving end, or with slow terminals or file systems, its possible for the event loop to be blocked often enough and long enough to have severe negative performance impacts. This may not be a problem when writing to an interactive terminal session, but consider this particularly careful when doing production logging to the process output streams.
To check if a stream is connected to a TTY context, check the isTTY
property.
For instance:
$ node -p "Boolean(process.stdin.isTTY)"
true
$ echo "foo" | node -p "Boolean(process.stdin.isTTY)"
false
$ node -p "Boolean(process.stdout.isTTY)"
true
$ node -p "Boolean(process.stdout.isTTY)" | cat
false
See the TTY documentation for more information.
process.title#
The process.title
property returns the current process title (i.e. returns
the current value of ps
). Assigning a new value to process.title
modifies
the current value of ps
.
Note: When a new value is assigned, different platforms will impose different
maximum length restrictions on the title. Usually such restrictions are quite
limited. For instance, on Linux and macOS, process.title
is limited to the
size of the binary name plus the length of the command line arguments because
setting the process.title
overwrites the argv
memory of the process.
Node.js v0.8 allowed for longer process title strings by also overwriting the
environ
memory but that was potentially insecure and confusing in some
(rather obscure) cases.
process.umask([mask])#
mask
<number>
The process.umask()
method sets or returns the Node.js process's file mode
creation mask. Child processes inherit the mask from the parent process. Invoked
without an argument, the current mask is returned, otherwise the umask is set to
the argument value and the previous mask is returned.
const newmask = 0o022;
const oldmask = process.umask(newmask);
console.log(
`Changed umask from ${oldmask.toString(8)} to ${newmask.toString(8)}`
);
process.uptime()#
- Returns: <number>
The process.uptime()
method returns the number of seconds the current Node.js
process has been running.
Note: the return value includes fractions of a second. Use Math.floor()
to get whole seconds.
process.version#
The process.version
property returns the Node.js version string.
console.log(`Version: ${process.version}`);
process.versions#
The process.versions
property returns an object listing the version strings of
Node.js and its dependencies. process.versions.modules
indicates the current
ABI version, which is increased whenever a C++ API changes. Node.js will refuse
to load modules that were compiled against a different module ABI version.
console.log(process.versions);
Will generate an object similar to:
{
http_parser: '2.3.0',
node: '1.1.1',
v8: '4.1.0.14',
uv: '1.3.0',
zlib: '1.2.8',
ares: '1.10.0-DEV',
modules: '43',
icu: '55.1',
openssl: '1.0.1k'
}
Exit Codes#
Node.js will normally exit with a 0
status code when no more async
operations are pending. The following status codes are used in other
cases:
1
Uncaught Fatal Exception - There was an uncaught exception, and it was not handled by a domain or an'uncaughtException'
event handler.2
- Unused (reserved by Bash for builtin misuse)3
Internal JavaScript Parse Error - The JavaScript source code internal in Node.js's bootstrapping process caused a parse error. This is extremely rare, and generally can only happen during development of Node.js itself.4
Internal JavaScript Evaluation Failure - The JavaScript source code internal in Node.js's bootstrapping process failed to return a function value when evaluated. This is extremely rare, and generally can only happen during development of Node.js itself.5
Fatal Error - There was a fatal unrecoverable error in V8. Typically a message will be printed to stderr with the prefixFATAL ERROR
.6
Non-function Internal Exception Handler - There was an uncaught exception, but the internal fatal exception handler function was somehow set to a non-function, and could not be called.7
Internal Exception Handler Run-Time Failure - There was an uncaught exception, and the internal fatal exception handler function itself threw an error while attempting to handle it. This can happen, for example, if a'uncaughtException'
ordomain.on('error')
handler throws an error.8
- Unused. In previous versions of Node.js, exit code 8 sometimes indicated an uncaught exception.9
- Invalid Argument - Either an unknown option was specified, or an option requiring a value was provided without a value.10
Internal JavaScript Run-Time Failure - The JavaScript source code internal in Node.js's bootstrapping process threw an error when the bootstrapping function was called. This is extremely rare, and generally can only happen during development of Node.js itself.12
Invalid Debug Argument - The--debug
,--inspect
and/or--debug-brk
options were set, but the port number chosen was invalid or unavailable.>128
Signal Exits - If Node.js receives a fatal signal such asSIGKILL
orSIGHUP
, then its exit code will be128
plus the value of the signal code. This is a standard POSIX practice, since exit codes are defined to be 7-bit integers, and signal exits set the high-order bit, and then contain the value of the signal code.