Node exit process with error

The process object provides information about, and control over, the current
Node.js process.

import process from 'node:process';

const process = require('node:process');

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.

import process from 'node:process';

process.on('beforeExit', (code) => {
  console.log('Process beforeExit event with code: ', code);
});

process.on('exit', (code) => {
  console.log('Process exit event with code: ', code);
});

console.log('This message is displayed first.');

// Prints:
// This message is displayed first.
// Process beforeExit event with code: 0
// Process exit event with code: 0

const process = require('node:process');

process.on('beforeExit', (code) => {
  console.log('Process beforeExit event with code: ', code);
});

process.on('exit', (code) => {
  console.log('Process exit event with code: ', code);
});

console.log('This message is displayed first.');

// Prints:
// This message is displayed first.
// Process beforeExit event with code: 0
// Process exit event with code: 0

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'

• code {integer}

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.

import process from 'node:process';

process.on('exit', (code) => {
  console.log(`About to exit with code: ${code}`);
});

const process = require('node:process');

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:

import process from 'node:process';

process.on('exit', (code) => {
  setTimeout(() => {
    console.log('This will not run');
  }, 0);
});

const process = require('node:process');

process.on('exit', (code) => {
  setTimeout(() => {
    console.log('This will not run');
  }, 0);
});

Event: 'message'

• message { Object | boolean | number | string | null } a parsed JSON object
  or a serializable primitive value.
• sendHandle {net.Server|net.Socket} a net.Server or net.Socket
  object, or undefined.

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 message goes through serialization and parsing. The resulting message might
not be the same as what is originally sent.

If the serialization option was set to advanced used when spawning the
process, the message argument can contain data that JSON is not able
to represent.
See Advanced serialization for child_process for more details.

Event: 'multipleResolves'

Stability: 0 — Deprecated

• type {string} The resolution type. One of 'resolve' or 'reject'.
• promise {Promise} The promise that resolved or rejected more than once.
• value {any} The value with which the promise was either resolved or
  rejected after the original resolve.

The 'multipleResolves' event is emitted whenever a Promise has been either:

• Resolved more than once.
• Rejected more than once.
• Rejected after resolve.
• Resolved after reject.

This is useful for tracking potential errors in an application while using the
Promise constructor, as multiple resolutions are silently swallowed. However,
the occurrence of this event does not necessarily indicate an error. For
example, Promise.race() can trigger a 'multipleResolves' event.

Because of the unreliability of the event in cases like the
Promise.race() example above it has been deprecated.

import process from 'node:process';

process.on('multipleResolves', (type, promise, reason) => {
  console.error(type, promise, reason);
  setImmediate(() => process.exit(1));
});

async function main() {
  try {
    return await new Promise((resolve, reject) => {
      resolve('First call');
      resolve('Swallowed resolve');
      reject(new Error('Swallowed reject'));
    });
  } catch {
    throw new Error('Failed');
  }
}

main().then(console.log);
// resolve: Promise { 'First call' } 'Swallowed resolve'
// reject: Promise { 'First call' } Error: Swallowed reject
//     at Promise (*)
//     at new Promise (<anonymous>)
//     at main (*)
// First call

const process = require('node:process');

process.on('multipleResolves', (type, promise, reason) => {
  console.error(type, promise, reason);
  setImmediate(() => process.exit(1));
});

async function main() {
  try {
    return await new Promise((resolve, reject) => {
      resolve('First call');
      resolve('Swallowed resolve');
      reject(new Error('Swallowed reject'));
    });
  } catch {
    throw new Error('Failed');
  }
}

main().then(console.log);
// resolve: Promise { 'First call' } 'Swallowed resolve'
// reject: Promise { 'First call' } Error: Swallowed reject
//     at Promise (*)
//     at new Promise (<anonymous>)
//     at main (*)
// First call

Event: 'rejectionHandled'

• promise {Promise} The late handled promise.

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 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.

import process from 'node:process';

const unhandledRejections = new Map();
process.on('unhandledRejection', (reason, promise) => {
  unhandledRejections.set(promise, reason);
});
process.on('rejectionHandled', (promise) => {
  unhandledRejections.delete(promise);
});

const process = require('node:process');

const unhandledRejections = new Map();
process.on('unhandledRejection', (reason, promise) => {
  unhandledRejections.set(promise, reason);
});
process.on('rejectionHandled', (promise) => {
  unhandledRejections.delete(promise);
});

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'

• err {Error} The uncaught exception.
• origin {string} Indicates if the exception originates from an unhandled
  rejection or from a synchronous error. Can either be 'uncaughtException' or
  'unhandledRejection'. The latter is used when an exception happens in a
  Promise based async context (or if a Promise is rejected) and
  --unhandled-rejections flag set to strict or throw (which is the
  default) and the rejection is not handled, or when a rejection happens during
  the command line entry point’s ES module static loading phase.

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
with code 1, overriding any previously set process.exitCode.
Adding a handler for the 'uncaughtException' event overrides this default
behavior. Alternatively, change the process.exitCode in the
'uncaughtException' handler which will result in the process exiting with the
provided exit code. Otherwise, in the presence of such handler the process will
exit with 0.

import process from 'node:process';

process.on('uncaughtException', (err, origin) => {
  fs.writeSync(
    process.stderr.fd,
    `Caught exception: ${err}n` +
    `Exception origin: ${origin}`,
  );
});

setTimeout(() => {
  console.log('This will still run.');
}, 500);

// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
console.log('This will not run.');

const process = require('node:process');

process.on('uncaughtException', (err, origin) => {
  fs.writeSync(
    process.stderr.fd,
    `Caught exception: ${err}n` +
    `Exception origin: ${origin}`,
  );
});

setTimeout(() => {
  console.log('This will still run.');
}, 500);

// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
console.log('This will not run.');

It is possible to monitor 'uncaughtException' events without overriding the
default behavior to exit the process by installing a
'uncaughtExceptionMonitor' listener.

Warning: Using 'uncaughtException' correctly

'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 the power cord when upgrading a computer. Nine out of ten
times, nothing happens. But the tenth 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: 'uncaughtExceptionMonitor'

• err {Error} The uncaught exception.
• origin {string} Indicates if the exception originates from an unhandled
  rejection or from synchronous errors. Can either be 'uncaughtException' or
  'unhandledRejection'. The latter is used when an exception happens in a
  Promise based async context (or if a Promise is rejected) and
  --unhandled-rejections flag set to strict or throw (which is the
  default) and the rejection is not handled, or when a rejection happens during
  the command line entry point’s ES module static loading phase.

The 'uncaughtExceptionMonitor' event is emitted before an
'uncaughtException' event is emitted or a hook installed via
process.setUncaughtExceptionCaptureCallback() is called.

Installing an 'uncaughtExceptionMonitor' listener does not change the behavior
once an 'uncaughtException' event is emitted. The process will
still crash if no 'uncaughtException' listener is installed.

import process from 'node:process';

process.on('uncaughtExceptionMonitor', (err, origin) => {
  MyMonitoringTool.logSync(err, origin);
});

// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
// Still crashes Node.js

const process = require('node:process');

process.on('uncaughtExceptionMonitor', (err, origin) => {
  MyMonitoringTool.logSync(err, origin);
});

// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
// Still crashes Node.js

Event: 'unhandledRejection'

• reason {Error|any} The object with which the promise was rejected
  (typically an Error object).
• promise {Promise} The rejected promise.

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.

import process from 'node:process';

process.on('unhandledRejection', (reason, promise) => {
  console.log('Unhandled Rejection at:', promise, '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()`

const process = require('node:process');

process.on('unhandledRejection', (reason, promise) => {
  console.log('Unhandled Rejection at:', promise, '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:

import process from 'node:process';

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

const process = require('node:process');

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.

Event: 'warning'

• warning {Error} Key properties of the warning are:
  • name {string} The name of the warning. Default: 'Warning'.
  • 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.

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.

import process from 'node:process';

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
});

const process = require('node:process');

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) MaxListenersExceededWarning: 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
> const 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.

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'.

Event: 'worker'

• worker {Worker} The {Worker} that was created.

The 'worker' event is emitted after a new {Worker} thread has been created.

Emitting custom warnings

See the process.emitWarning() method for issuing
custom or application-specific warnings.

Node.js warning names

There are no strict guidelines for warning types (as identified by the name
property) emitted by Node.js. New types of warnings can be added at any time.
A few of the warning types that are most common include:

• 'DeprecationWarning' — Indicates use of a deprecated Node.js API or feature.
  Such warnings must include a 'code' property identifying the
  deprecation code.
• 'ExperimentalWarning' — Indicates use of an experimental Node.js API or
  feature. Such features must be used with caution as they may change at any
  time and are not subject to the same strict semantic-versioning and long-term
  support policies as supported features.
• 'MaxListenersExceededWarning' — Indicates that too many listeners for a
  given event have been registered on either an EventEmitter or EventTarget.
  This is often an indication of a memory leak.
• 'TimeoutOverflowWarning' — Indicates that a numeric value that cannot fit
  within a 32-bit signed integer has been provided to either the setTimeout()
  or setInterval() functions.
• 'UnsupportedWarning' — Indicates use of an unsupported option or feature
  that will be ignored rather than treated as an error. One example is use of
  the HTTP response status message when using the HTTP/2 compatibility API.

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.

Signals are not available on Worker threads.

The signal handler will receive the signal’s name ('SIGINT',
'SIGTERM', etc.) as the first argument.

The name of each event will be the uppercase common name for the signal (e.g.
'SIGINT' for SIGINT signals).

import process from 'node:process';

// 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.');
});

// Using a single function to handle multiple signals
function handle(signal) {
  console.log(`Received ${signal}`);
}

process.on('SIGINT', handle);
process.on('SIGTERM', handle);

const process = require('node:process');

// 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.');
});

// Using a single function to handle multiple signals
function handle(signal) {
  console.log(`Received ${signal}`);
}

process.on('SIGINT', handle);
process.on('SIGTERM', handle);

• 'SIGUSR1' is reserved by Node.js to start the debugger. It’s possible to
  install a listener but doing so might interfere with the debugger.
• 'SIGTERM' and 'SIGINT' have default handlers on non-Windows platforms that
  reset the terminal mode before exiting with code 128 + 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 of SIGHUP 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 with Ctrl+C (though this may be configurable).
  It is not generated when terminal raw mode is enabled
  and Ctrl+C is used.
• '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', and 'SIGILL', when not raised
  artificially using kill(2), inherently leave the process in a state from
  which it is not safe to call JS listeners. Doing so might cause the process
  to stop responding.
• 0 can be sent to test for the existence of a process, it has no effect if
  the process exists, but will throw an error if the process does not exist.

Windows does not support signals so has no equivalent to termination by signal,
but Node.js offers some emulation with process.kill(), and
subprocess.kill():

• Sending SIGINT, SIGTERM, and SIGKILL will cause the unconditional
  termination of the target process, and afterwards, subprocess will report that
  the process was terminated by signal.
• Sending signal 0 can be used as a platform independent way to test for the
  existence of a process.

process.abort()

The process.abort() method causes the Node.js process to exit immediately and
generate a core file.

This feature is not available in Worker threads.

process.allowedNodeEnvironmentFlags

• {Set}

The process.allowedNodeEnvironmentFlags property is a special,
read-only Set of flags allowable within the NODE_OPTIONS
environment variable.

process.allowedNodeEnvironmentFlags extends Set, but overrides
Set.prototype.has to recognize several different possible flag
representations. process.allowedNodeEnvironmentFlags.has() will
return true in the following cases:

• Flags may omit leading single (-) or double (--) dashes; e.g.,
  inspect-brk for --inspect-brk, or r for -r.
• Flags passed through to V8 (as listed in --v8-options) may replace
  one or more non-leading dashes for an underscore, or vice-versa;
  e.g., --perf_basic_prof, --perf-basic-prof, --perf_basic-prof,
  etc.
• Flags may contain one or more equals (=) characters; all
  characters after and including the first equals will be ignored;
  e.g., --stack-trace-limit=100.
• Flags must be allowable within NODE_OPTIONS.

When iterating over process.allowedNodeEnvironmentFlags, flags will
appear only once; each will begin with one or more dashes. Flags
passed through to V8 will contain underscores instead of non-leading
dashes:

import { allowedNodeEnvironmentFlags } from 'node:process';

allowedNodeEnvironmentFlags.forEach((flag) => {
  // -r
  // --inspect-brk
  // --abort_on_uncaught_exception
  // ...
});

const { allowedNodeEnvironmentFlags } = require('node:process');

allowedNodeEnvironmentFlags.forEach((flag) => {
  // -r
  // --inspect-brk
  // --abort_on_uncaught_exception
  // ...
});

The methods add(), clear(), and delete() of
process.allowedNodeEnvironmentFlags do nothing, and will fail
silently.

If Node.js was compiled without NODE_OPTIONS support (shown in
process.config), process.allowedNodeEnvironmentFlags will
contain what would have been allowable.

process.arch

• {string}

The operating system CPU architecture for which the Node.js binary was compiled.
Possible values are: 'arm', 'arm64', 'ia32', 'mips','mipsel', 'ppc',
'ppc64', 's390', 's390x', and 'x64'.

import { arch } from 'node:process';

console.log(`This processor architecture is ${arch}`);

const { arch } = require('node:process');

console.log(`This processor architecture is ${arch}`);

process.argv

• {string[]}

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:

import { argv } from 'node:process';

// print process.argv
argv.forEach((val, index) => {
  console.log(`${index}: ${val}`);
});

const { argv } = require('node:process');

// print process.argv
argv.forEach((val, index) => {
  console.log(`${index}: ${val}`);
});

Launching the Node.js process as:

$ node process-args.js one two=three four

Would generate the output:

0: /usr/local/bin/node
1: /Users/mjr/work/node/process-args.js
2: one
3: two=three
4: four

process.argv0

• {string}

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.channel

• {Object}

If the Node.js process was spawned with an IPC channel (see the
Child Process documentation), the process.channel
property is a reference to the IPC channel. If no IPC channel exists, this
property is undefined.

process.channel.ref()

This method makes the IPC channel keep the event loop of the process
running if .unref() has been called before.

Typically, this is managed through the number of 'disconnect' and 'message'
listeners on the process object. However, this method can be used to
explicitly request a specific behavior.

process.channel.unref()

This method makes the IPC channel not keep the event loop of the process
running, and lets it finish even while the channel is open.

Typically, this is managed through the number of 'disconnect' and 'message'
listeners on the process object. However, this method can be used to
explicitly request a specific behavior.

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).

import { chdir, cwd } from 'node:process';

console.log(`Starting directory: ${cwd()}`);
try {
  chdir('/tmp');
  console.log(`New directory: ${cwd()}`);
} catch (err) {
  console.error(`chdir: ${err}`);
}

const { chdir, cwd } = require('node:process');

console.log(`Starting directory: ${cwd()}`);
try {
  chdir('/tmp');
  console.log(`New directory: ${cwd()}`);
} catch (err) {
  console.error(`chdir: ${err}`);
}

This feature is not available in Worker threads.

process.config

• {Object}

The process.config property returns a frozen 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',
     napi_build_version: 5,
     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_openssl: 'true',
     node_shared_openssl: 'false',
     strict_aliasing: 'true',
     target_arch: 'x64',
     v8_use_snapshot: 1
   }
}

process.connected

• {boolean}

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.constrainedMemory()

Stability: 1 — Experimental

• {number|undefined}

Gets the amount of memory available to the process (in bytes) based on
limits imposed by the OS. If there is no such constraint, or the constraint
is unknown, undefined is returned.

See uv_get_constrained_memory for more
information.

process.cpuUsage([previousValue])

• previousValue {Object} A previous return value from calling
  process.cpuUsage()
• Returns: {Object}
  • user {integer}
  • system {integer}

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.

import { cpuUsage } from 'node:process';

const startUsage = cpuUsage();
// { user: 38579, system: 6986 }

// spin the CPU for 500 milliseconds
const now = Date.now();
while (Date.now() - now < 500);

console.log(cpuUsage(startUsage));
// { user: 514883, system: 11226 }

const { cpuUsage } = require('node:process');

const startUsage = cpuUsage();
// { user: 38579, system: 6986 }

// spin the CPU for 500 milliseconds
const now = Date.now();
while (Date.now() - now < 500);

console.log(cpuUsage(startUsage));
// { user: 514883, system: 11226 }

process.cwd()

• Returns: {string}

The process.cwd() method returns the current working directory of the Node.js
process.

import { cwd } from 'node:process';

console.log(`Current directory: ${cwd()}`);

const { cwd } = require('node:process');

console.log(`Current directory: ${cwd()}`);

process.debugPort

• {number}

The port used by the Node.js debugger when enabled.

import process from 'node:process';

process.debugPort = 5858;

const process = require('node:process');

process.debugPort = 5858;

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 the same as calling
ChildProcess.disconnect() from the parent process.

If the Node.js process was not spawned with an IPC channel,
process.disconnect() will be undefined.

process.dlopen(module, filename[, flags])

• module {Object}
• filename {string}
• flags {os.constants.dlopen} Default: os.constants.dlopen.RTLD_LAZY

The process.dlopen() method allows dynamically loading shared objects. It is
primarily used by require() to load C++ Addons, and should not be used
directly, except in special cases. In other words, require() should be
preferred over process.dlopen() unless there are specific reasons such as
custom dlopen flags or loading from ES modules.

The flags argument is an integer that allows to specify dlopen
behavior. See the os.constants.dlopen documentation for details.

An important requirement when calling process.dlopen() is that the module
instance must be passed. Functions exported by the C++ Addon are then
accessible via module.exports.

The example below shows how to load a C++ Addon, named local.node,
that exports a foo function. All the symbols are loaded before
the call returns, by passing the RTLD_NOW constant. In this example
the constant is assumed to be available.

import { dlopen } from 'node:process';
import { constants } from 'node:os';
import { fileURLToPath } from 'node:url';

const module = { exports: {} };
dlopen(module, fileURLToPath(new URL('local.node', import.meta.url)),
       constants.dlopen.RTLD_NOW);
module.exports.foo();

const { dlopen } = require('node:process');
const { constants } = require('node:os');
const { join } = require('node:path');

const module = { exports: {} };
dlopen(module, join(__dirname, 'local.node'), constants.dlopen.RTLD_NOW);
module.exports.foo();

process.emitWarning(warning[, options])

• warning {string|Error} The warning to emit.
• options {Object}
  • type {string} When warning is a String, type is the name to use
    for the type of warning being emitted. Default: 'Warning'.
  • code {string} A unique identifier for the warning instance being emitted.
  • ctor {Function} When warning is a String, ctor is an optional
    function used to limit the generated stack trace. Default:
    process.emitWarning.
  • detail {string} Additional text to include with the error.

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
'warning' event.

import { emitWarning } from 'node:process';

// Emit a warning with a code and additional detail.
emitWarning('Something happened!', {
  code: 'MY_WARNING',
  detail: 'This is some additional information',
});
// Emits:
// (node:56338) [MY_WARNING] Warning: Something happened!
// This is some additional information

const { emitWarning } = require('node:process');

// Emit a warning with a code and additional detail.
emitWarning('Something happened!', {
  code: 'MY_WARNING',
  detail: 'This is some additional information',
});
// Emits:
// (node:56338) [MY_WARNING] Warning: Something happened!
// This is some additional information

In this example, an Error object is generated internally by
process.emitWarning() and passed through to the
'warning' handler.

import process from 'node:process';

process.on('warning', (warning) => {
  console.warn(warning.name);    // 'Warning'
  console.warn(warning.message); // 'Something happened!'
  console.warn(warning.code);    // 'MY_WARNING'
  console.warn(warning.stack);   // Stack trace
  console.warn(warning.detail);  // 'This is some additional information'
});

const process = require('node:process');

process.on('warning', (warning) => {
  console.warn(warning.name);    // 'Warning'
  console.warn(warning.message); // 'Something happened!'
  console.warn(warning.code);    // 'MY_WARNING'
  console.warn(warning.stack);   // Stack trace
  console.warn(warning.detail);  // 'This is some additional information'
});

If warning is passed as an Error object, the options argument is ignored.

process.emitWarning(warning[, type[, code]][, ctor])

• warning {string|Error} The warning to emit.
• type {string} When warning is a String, type is the name to use
  for the type of warning being emitted. Default: 'Warning'.
• code {string} A unique identifier for the warning instance being emitted.
• ctor {Function} When warning is a String, ctor is an optional
  function used to limit the generated stack trace. Default:
  process.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
'warning' event.

import { emitWarning } from 'node:process';

// Emit a warning using a string.
emitWarning('Something happened!');
// Emits: (node: 56338) Warning: Something happened!

const { emitWarning } = require('node:process');

// Emit a warning using a string.
emitWarning('Something happened!');
// Emits: (node: 56338) Warning: Something happened!

import { emitWarning } from 'node:process';

// Emit a warning using a string and a type.
emitWarning('Something Happened!', 'CustomWarning');
// Emits: (node:56338) CustomWarning: Something Happened!

const { emitWarning } = require('node:process');

// Emit a warning using a string and a type.
emitWarning('Something Happened!', 'CustomWarning');
// Emits: (node:56338) CustomWarning: Something Happened!

import { emitWarning } from 'node:process';

emitWarning('Something happened!', 'CustomWarning', 'WARN001');
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!

const { emitWarning } = require('node:process');

process.emitWarning('Something happened!', 'CustomWarning', 'WARN001');
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!

In each of the previous examples, an Error object is generated internally by
process.emitWarning() and passed through to the 'warning'
handler.

import process from 'node:process';

process.on('warning', (warning) => {
  console.warn(warning.name);
  console.warn(warning.message);
  console.warn(warning.code);
  console.warn(warning.stack);
});

const process = require('node:process');

process.on('warning', (warning) => {
  console.warn(warning.name);
  console.warn(warning.message);
  console.warn(warning.code);
  console.warn(warning.stack);
});

If warning is passed as an Error object, it will be passed through to the
'warning' event handler unmodified (and the optional type,
code and ctor arguments will be ignored):

import { emitWarning } from 'node:process';

// Emit a warning using an Error object.
const myWarning = new Error('Something happened!');
// Use the Error name property to specify the type name
myWarning.name = 'CustomWarning';
myWarning.code = 'WARN001';

emitWarning(myWarning);
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!

const { emitWarning } = require('node:process');

// Emit a warning using an Error object.
const myWarning = new Error('Something happened!');
// Use the Error name property to specify the type name
myWarning.name = 'CustomWarning';
myWarning.code = 'WARN001';

emitWarning(myWarning);
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!

A TypeError is thrown if warning is anything other than a string or Error
object.

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 type 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 to stderr along with the full stack trace.

Avoiding duplicate warnings

As a best practice, warnings should be emitted only once per process. To do
so, place the emitWarning() behind a boolean.

import { emitWarning } from 'node:process';

function emitMyWarning() {
  if (!emitMyWarning.warned) {
    emitMyWarning.warned = true;
    emitWarning('Only warn once!');
  }
}
emitMyWarning();
// Emits: (node: 56339) Warning: Only warn once!
emitMyWarning();
// Emits nothing

const { emitWarning } = require('node:process');

function emitMyWarning() {
  if (!emitMyWarning.warned) {
    emitMyWarning.warned = true;
    emitWarning('Only warn once!');
  }
}
emitMyWarning();
// Emits: (node: 56339) Warning: Only warn once!
emitMyWarning();
// Emits nothing

process.env

• {Object}

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, or (unless explicitly requested)
to other Worker threads.
In other words, the following example would not work:

$ node -e 'process.env.foo = "bar"' && echo $foo

While the following will:

import { env } from 'node:process';

env.foo = 'bar';
console.log(env.foo);

const { env } = require('node:process');

env.foo = 'bar';
console.log(env.foo);

Assigning a property on process.env will implicitly convert the value
to a string. This behavior is deprecated. Future versions of Node.js may
throw an error when the value is not a string, number, or boolean.

import { env } from 'node:process';

env.test = null;
console.log(env.test);
// => 'null'
env.test = undefined;
console.log(env.test);
// => 'undefined'

const { env } = require('node:process');

env.test = null;
console.log(env.test);
// => 'null'
env.test = undefined;
console.log(env.test);
// => 'undefined'

Use delete to delete a property from process.env.

import { env } from 'node:process';

env.TEST = 1;
delete env.TEST;
console.log(env.TEST);
// => undefined

const { env } = require('node:process');

env.TEST = 1;
delete env.TEST;
console.log(env.TEST);
// => undefined

On Windows operating systems, environment variables are case-insensitive.

import { env } from 'node:process';

env.TEST = 1;
console.log(env.test);
// => 1

const { env } = require('node:process');

env.TEST = 1;
console.log(env.test);
// => 1

Unless explicitly specified when creating a Worker instance,
each Worker thread has its own copy of process.env, based on its
parent thread’s process.env, or whatever was specified as the env option
to the Worker constructor. Changes to process.env will not be visible
across Worker threads, and only the main thread can make changes that
are visible to the operating system or to native add-ons.

process.execArgv

• {string[]}

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.

$ node --harmony script.js --version

Results in process.execArgv:

And process.argv:

['/usr/local/bin/node', 'script.js', '--version']

Refer to Worker constructor for the detailed behavior of worker
threads with this property.

process.execPath

• {string}

The process.execPath property returns the absolute pathname of the executable
that started the Node.js process. Symbolic links, if any, are resolved.

process.exit([code])

• code {integer|string|null|undefined} The exit code. For string type, only
  integer strings (e.g.,’1′) are allowed. Default: 0.

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:

import { exit } from 'node:process';

exit(1);

const { exit } = require('node:process');

exit(1);

The shell that executed Node.js should see the exit code as 1.

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:

import { exit } from 'node:process';

// This is an example of what *not* to do:
if (someConditionNotMet()) {
  printUsageToStdout();
  exit(1);
}

const { exit } = require('node:process');

// This is an example of what *not* to do:
if (someConditionNotMet()) {
  printUsageToStdout();
  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:

import process from 'node:process';

// How to properly set the exit code while letting
// the process exit gracefully.
if (someConditionNotMet()) {
  printUsageToStdout();
  process.exitCode = 1;
}

const process = require('node:process');

// 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().

In Worker threads, this function stops the current thread rather
than the current process.

process.exitCode

• {integer|string|null|undefined} The exit code. For string type, only
  integer strings (e.g.,’1′) are allowed. Default: undefined.

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.getActiveResourcesInfo()

Stability: 1 — Experimental

• Returns: {string[]}

The process.getActiveResourcesInfo() method returns an array of strings
containing the types of the active resources that are currently keeping the
event loop alive.

import { getActiveResourcesInfo } from 'node:process';
import { setTimeout } from 'node:timers';

console.log('Before:', getActiveResourcesInfo());
setTimeout(() => {}, 1000);
console.log('After:', getActiveResourcesInfo());
// Prints:
//   Before: [ 'CloseReq', 'TTYWrap', 'TTYWrap', 'TTYWrap' ]
//   After: [ 'CloseReq', 'TTYWrap', 'TTYWrap', 'TTYWrap', 'Timeout' ]

const { getActiveResourcesInfo } = require('node:process');
const { setTimeout } = require('node:timers');

console.log('Before:', getActiveResourcesInfo());
setTimeout(() => {}, 1000);
console.log('After:', getActiveResourcesInfo());
// Prints:
//   Before: [ 'TTYWrap', 'TTYWrap', 'TTYWrap' ]
//   After: [ 'TTYWrap', 'TTYWrap', 'TTYWrap', 'Timeout' ]

process.getegid()

The process.getegid() method returns the numerical effective group identity
of the Node.js process. (See getegid(2).)

import process from 'node:process';

if (process.getegid) {
  console.log(`Current gid: ${process.getegid()}`);
}

const process = require('node:process');

if (process.getegid) {
  console.log(`Current gid: ${process.getegid()}`);
}

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).)

import process from 'node:process';

if (process.geteuid) {
  console.log(`Current uid: ${process.geteuid()}`);
}

const process = require('node:process');

if (process.geteuid) {
  console.log(`Current uid: ${process.geteuid()}`);
}

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).)

import process from 'node:process';

if (process.getgid) {
  console.log(`Current gid: ${process.getgid()}`);
}

const process = require('node:process');

if (process.getgid) {
  console.log(`Current gid: ${process.getgid()}`);
}

This function is only available on POSIX platforms (i.e. not Windows or
Android).

process.getgroups()

• Returns: {integer[]}

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.

import process from 'node:process';

if (process.getgroups) {
  console.log(process.getgroups()); // [ 16, 21, 297 ]
}

const process = require('node:process');

if (process.getgroups) {
  console.log(process.getgroups()); // [ 16, 21, 297 ]
}

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).)

import process from 'node:process';

if (process.getuid) {
  console.log(`Current uid: ${process.getuid()}`);
}

const process = require('node:process');

if (process.getuid) {
  console.log(`Current uid: ${process.getuid()}`);
}

This function is only available on POSIX platforms (i.e. not Windows or
Android).

process.hasUncaughtExceptionCaptureCallback()

• Returns: {boolean}

Indicates whether a callback has been set using
process.setUncaughtExceptionCaptureCallback().

process.hrtime([time])

Stability: 3 — Legacy. Use process.hrtime.bigint() instead.

• time {integer[]} The result of a previous call to process.hrtime()
• Returns: {integer[]}

This is the legacy version of process.hrtime.bigint()
before bigint was introduced in JavaScript.

The process.hrtime() method returns the current high-resolution real time
in a [seconds, nanoseconds] tuple Array, where nanoseconds is the
remaining part of the real time that can’t be represented in second precision.

time is an optional parameter that must be the result of a previous
process.hrtime() call to diff with the current time. If the parameter
passed in is not a tuple Array, a TypeError will be thrown. Passing in a
user-defined array instead of the result of a previous call to
process.hrtime() will lead to undefined behavior.

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:

import { hrtime } from 'node:process';

const NS_PER_SEC = 1e9;
const time = hrtime();
// [ 1800216, 25 ]

setTimeout(() => {
  const diff = hrtime(time);
  // [ 1, 552 ]

  console.log(`Benchmark took ${diff[0] * NS_PER_SEC + diff[1]} nanoseconds`);
  // Benchmark took 1000000552 nanoseconds
}, 1000);

const { hrtime } = require('node:process');

const NS_PER_SEC = 1e9;
const time = hrtime();
// [ 1800216, 25 ]

setTimeout(() => {
  const diff = hrtime(time);
  // [ 1, 552 ]

  console.log(`Benchmark took ${diff[0] * NS_PER_SEC + diff[1]} nanoseconds`);
  // Benchmark took 1000000552 nanoseconds
}, 1000);

process.hrtime.bigint()

• Returns: {bigint}

The bigint version of the process.hrtime() method returning the
current high-resolution real time in nanoseconds as a bigint.

Unlike process.hrtime(), it does not support an additional time
argument since the difference can just be computed directly
by subtraction of the two bigints.

import { hrtime } from 'node:process';

const start = hrtime.bigint();
// 191051479007711n

setTimeout(() => {
  const end = hrtime.bigint();
  // 191052633396993n

  console.log(`Benchmark took ${end - start} nanoseconds`);
  // Benchmark took 1154389282 nanoseconds
}, 1000);

const { hrtime } = require('node:process');

const start = hrtime.bigint();
// 191051479007711n

setTimeout(() => {
  const end = hrtime.bigint();
  // 191052633396993n

  console.log(`Benchmark took ${end - start} nanoseconds`);
  // Benchmark took 1154389282 nanoseconds
}, 1000);

process.initgroups(user, extraGroup)

• user {string|number} The user name or numeric identifier.
• extraGroup {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.

Use care when dropping privileges:

import { getgroups, initgroups, setgid } from 'node:process';

console.log(getgroups());         // [ 0 ]
initgroups('nodeuser', 1000);     // switch user
console.log(getgroups());         // [ 27, 30, 46, 1000, 0 ]
setgid(1000);                     // drop root gid
console.log(getgroups());         // [ 27, 30, 46, 1000 ]

const { getgroups, initgroups, setgid } = require('node:process');

console.log(getgroups());         // [ 0 ]
initgroups('nodeuser', 1000);     // switch user
console.log(getgroups());         // [ 27, 30, 46, 1000, 0 ]
setgid(1000);                     // drop root gid
console.log(getgroups());         // [ 27, 30, 46, 1000 ]

This function is only available on POSIX platforms (i.e. not Windows or
Android).
This feature is not available in Worker threads.

process.kill(pid[, signal])

• pid {number} A process ID
• signal {string|number} The signal to send, either as a string or number.
  Default: '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.

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.

import process, { kill } from 'node:process';

process.on('SIGHUP', () => {
  console.log('Got SIGHUP signal.');
});

setTimeout(() => {
  console.log('Exiting.');
  process.exit(0);
}, 100);

kill(process.pid, 'SIGHUP');

const process = require('node:process');

process.on('SIGHUP', () => {
  console.log('Got SIGHUP signal.');
});

setTimeout(() => {
  console.log('Exiting.');
  process.exit(0);
}, 100);

process.kill(process.pid, 'SIGHUP');

When SIGUSR1 is received by a Node.js process, Node.js will start the
debugger. See Signal Events.

process.mainModule

Stability: 0 — Deprecated: Use require.main instead.

• {Object}

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()

• Returns: {Object}
  • rss {integer}
  • heapTotal {integer}
  • heapUsed {integer}
  • external {integer}
  • arrayBuffers {integer}

Returns an object describing the memory usage of the Node.js process measured in
bytes.

import { memoryUsage } from 'node:process';

console.log(memoryUsage());
// Prints:
// {
//  rss: 4935680,
//  heapTotal: 1826816,
//  heapUsed: 650472,
//  external: 49879,
//  arrayBuffers: 9386
// }

const { memoryUsage } = require('node:process');

console.log(memoryUsage());
// Prints:
// {
//  rss: 4935680,
//  heapTotal: 1826816,
//  heapUsed: 650472,
//  external: 49879,
//  arrayBuffers: 9386
// }

• 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, including all C++ and JavaScript objects and code.
• arrayBuffers refers to memory allocated for ArrayBuffers and
  SharedArrayBuffers, including all Node.js Buffers.
  This is also included in the external value. When Node.js is used as an
  embedded library, this value may be 0 because allocations for ArrayBuffers
  may not be tracked in that case.

When using Worker threads, rss will be a value that is valid for the
entire process, while the other fields will only refer to the current thread.

The process.memoryUsage() method iterates over each page to gather
information about memory usage which might be slow depending on the
program memory allocations.

process.memoryUsage.rss()

• Returns: {integer}

The process.memoryUsage.rss() method returns an integer representing the
Resident Set Size (RSS) in bytes.

The 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, including all C++ and JavaScript objects and code.

This is the same value as the rss property provided by process.memoryUsage()
but process.memoryUsage.rss() is faster.

import { memoryUsage } from 'node:process';

console.log(memoryUsage.rss());
// 35655680

const { rss } = require('node:process');

console.log(memoryUsage.rss());
// 35655680

process.nextTick(callback[, ...args])

• callback {Function}
• ...args {any} Additional arguments to pass when invoking the callback

process.nextTick() adds callback to the «next tick queue». This queue is
fully drained after the current operation on the JavaScript stack runs to
completion and before the event loop is allowed to continue. It’s possible to
create an infinite loop if one were to recursively call process.nextTick().
See the Event Loop guide for more background.

import { nextTick } from 'node:process';

console.log('start');
nextTick(() => {
  console.log('nextTick callback');
});
console.log('scheduled');
// Output:
// start
// scheduled
// nextTick callback

const { nextTick } = require('node:process');

console.log('start');
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:

import { nextTick } from 'node:process';

function MyThing(options) {
  this.setupOptions(options);

  nextTick(() => {
    this.startDoingStuff();
  });
}

const thing = new MyThing();
thing.getReadyForStuff();

// thing.startDoingStuff() gets called now, not before.

const { nextTick } = require('node:process');

function MyThing(options) {
  this.setupOptions(options);

  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:

import { nextTick } from 'node:process';

function definitelyAsync(arg, cb) {
  if (arg) {
    nextTick(cb);
    return;
  }

  fs.stat('file', cb);
}

const { nextTick } = require('node:process');

function definitelyAsync(arg, cb) {
  if (arg) {
    nextTick(cb);
    return;
  }

  fs.stat('file', cb);
}

When to use queueMicrotask() vs. process.nextTick()

The queueMicrotask() API is an alternative to process.nextTick() that
also defers execution of a function using the same microtask queue used to
execute the then, catch, and finally handlers of resolved promises. Within
Node.js, every time the «next tick queue» is drained, the microtask queue
is drained immediately after.

import { nextTick } from 'node:process';

Promise.resolve().then(() => console.log(2));
queueMicrotask(() => console.log(3));
nextTick(() => console.log(1));
// Output:
// 1
// 2
// 3

const { nextTick } = require('node:process');

Promise.resolve().then(() => console.log(2));
queueMicrotask(() => console.log(3));
nextTick(() => console.log(1));
// Output:
// 1
// 2
// 3

For most userland use cases, the queueMicrotask() API provides a portable
and reliable mechanism for deferring execution that works across multiple
JavaScript platform environments and should be favored over process.nextTick().
In simple scenarios, queueMicrotask() can be a drop-in replacement for
process.nextTick().

console.log('start');
queueMicrotask(() => {
  console.log('microtask callback');
});
console.log('scheduled');
// Output:
// start
// scheduled
// microtask callback

One note-worthy difference between the two APIs is that process.nextTick()
allows specifying additional values that will be passed as arguments to the
deferred function when it is called. Achieving the same result with
queueMicrotask() requires using either a closure or a bound function:

function deferred(a, b) {
  console.log('microtask', a + b);
}

console.log('start');
queueMicrotask(deferred.bind(undefined, 1, 2));
console.log('scheduled');
// Output:
// start
// scheduled
// microtask 3

There are minor differences in the way errors raised from within the next tick
queue and microtask queue are handled. Errors thrown within a queued microtask
callback should be handled within the queued callback when possible. If they are
not, the process.on('uncaughtException') event handler can be used to capture
and handle the errors.

When in doubt, unless the specific capabilities of process.nextTick() are
needed, use queueMicrotask().

process.noDeprecation

• {boolean}

The process.noDeprecation property indicates whether the --no-deprecation
flag is set on the current Node.js process. See the documentation for
the 'warning' event and the
emitWarning() method for more information about this
flag’s behavior.

process.pid

• {integer}

The process.pid property returns the PID of the process.

import { pid } from 'node:process';

console.log(`This process is pid ${pid}`);

const { pid } = require('node:process');

console.log(`This process is pid ${pid}`);

process.platform

• {string}

The process.platform property returns a string identifying the operating
system platform for which the Node.js binary was compiled.

Currently possible values are:

• 'aix'
• 'darwin'
• 'freebsd'
• 'linux'
• 'openbsd'
• 'sunos'
• 'win32'

import { platform } from 'node:process';

console.log(`This platform is ${platform}`);

const { platform } = require('node:process');

console.log(`This platform is ${platform}`);

The value 'android' may also be returned if the Node.js is built on the
Android operating system. However, Android support in Node.js
is experimental.

process.ppid

• {integer}

The process.ppid property returns the PID of the parent of the
current process.

import { ppid } from 'node:process';

console.log(`The parent process is pid ${ppid}`);

const { ppid } = require('node:process');

console.log(`The parent process is pid ${ppid}`);

process.release

• {Object}

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'.
• 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|undefined} an absolute URL pointing to a node.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|undefined} a string label identifying the LTS label for this
  release. This property only exists for LTS releases and is undefined for all
  other release types, including Current releases. Valid values include the
  LTS Release code names (including those that are no longer supported).
  • 'Fermium' for the 14.x LTS line beginning with 14.15.0.
  • 'Gallium' for the 16.x LTS line beginning with 16.13.0.
  • 'Hydrogen' for the 18.x LTS line beginning with 18.12.0.
    For other LTS Release code names, see Node.js Changelog Archive

{
  name: 'node',
  lts: 'Hydrogen',
  sourceUrl: 'https://nodejs.org/download/release/v18.12.0/node-v18.12.0.tar.gz',
  headersUrl: 'https://nodejs.org/download/release/v18.12.0/node-v18.12.0-headers.tar.gz',
  libUrl: 'https://nodejs.org/download/release/v18.12.0/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.report

• {Object}

process.report is an object whose methods are used to generate diagnostic
reports for the current process. Additional documentation is available in the
report documentation.

process.report.compact

• {boolean}

Write reports in a compact format, single-line JSON, more easily consumable
by log processing systems than the default multi-line format designed for
human consumption.

import { report } from 'node:process';

console.log(`Reports are compact? ${report.compact}`);

const { report } = require('node:process');

console.log(`Reports are compact? ${report.compact}`);

process.report.directory

• {string}

Directory where the report is written. The default value is the empty string,
indicating that reports are written to the current working directory of the
Node.js process.

import { report } from 'node:process';

console.log(`Report directory is ${report.directory}`);

const { report } = require('node:process');

console.log(`Report directory is ${report.directory}`);

process.report.filename

• {string}

Filename where the report is written. If set to the empty string, the output
filename will be comprised of a timestamp, PID, and sequence number. The default
value is the empty string.

If the value of process.report.filename is set to 'stdout' or 'stderr',
the report is written to the stdout or stderr of the process respectively.

import { report } from 'node:process';

console.log(`Report filename is ${report.filename}`);

const { report } = require('node:process');

console.log(`Report filename is ${report.filename}`);

process.report.getReport([err])

• err {Error} A custom error used for reporting the JavaScript stack.
• Returns: {Object}

Returns a JavaScript Object representation of a diagnostic report for the
running process. The report’s JavaScript stack trace is taken from err, if
present.

import { report } from 'node:process';

const data = report.getReport();
console.log(data.header.nodejsVersion);

// Similar to process.report.writeReport()
import fs from 'node:fs';
fs.writeFileSync('my-report.log', util.inspect(data), 'utf8');

const { report } = require('node:process');

const data = report.getReport();
console.log(data.header.nodejsVersion);

// Similar to process.report.writeReport()
const fs = require('node:fs');
fs.writeFileSync('my-report.log', util.inspect(data), 'utf8');

Additional documentation is available in the report documentation.

process.report.reportOnFatalError

• {boolean}

If true, a diagnostic report is generated on fatal errors, such as out of
memory errors or failed C++ assertions.

import { report } from 'node:process';

console.log(`Report on fatal error: ${report.reportOnFatalError}`);

const { report } = require('node:process');

console.log(`Report on fatal error: ${report.reportOnFatalError}`);

process.report.reportOnSignal

• {boolean}

If true, a diagnostic report is generated when the process receives the
signal specified by process.report.signal.

import { report } from 'node:process';

console.log(`Report on signal: ${report.reportOnSignal}`);

const { report } = require('node:process');

console.log(`Report on signal: ${report.reportOnSignal}`);

process.report.reportOnUncaughtException

• {boolean}

If true, a diagnostic report is generated on uncaught exception.

import { report } from 'node:process';

console.log(`Report on exception: ${report.reportOnUncaughtException}`);

const { report } = require('node:process');

console.log(`Report on exception: ${report.reportOnUncaughtException}`);

process.report.signal

• {string}

The signal used to trigger the creation of a diagnostic report. Defaults to
'SIGUSR2'.

import { report } from 'node:process';

console.log(`Report signal: ${report.signal}`);

const { report } = require('node:process');

console.log(`Report signal: ${report.signal}`);

process.report.writeReport([filename][, err])

• filename {string} Name of the file where the report is written. This
  should be a relative path, that will be appended to the directory specified in
  process.report.directory, or the current working directory of the Node.js
  process, if unspecified.

• err {Error} A custom error used for reporting the JavaScript stack.

• Returns: {string} Returns the filename of the generated report.

Writes a diagnostic report to a file. If filename is not provided, the default
filename includes the date, time, PID, and a sequence number. The report’s
JavaScript stack trace is taken from err, if present.

If the value of filename is set to 'stdout' or 'stderr', the report is
written to the stdout or stderr of the process respectively.

import { report } from 'node:process';

report.writeReport();

const { report } = require('node:process');

report.writeReport();

Additional documentation is available in the report documentation.

process.resourceUsage()

• Returns: {Object} the resource usage for the current process. All of these
  values come from the uv_getrusage call which returns
  a uv_rusage_t struct.
  • userCPUTime {integer} maps to ru_utime computed in microseconds.
    It is the same value as process.cpuUsage().user.
  • systemCPUTime {integer} maps to ru_stime computed in microseconds.
    It is the same value as process.cpuUsage().system.
  • maxRSS {integer} maps to ru_maxrss which is the maximum resident set
    size used in kilobytes.
  • sharedMemorySize {integer} maps to ru_ixrss but is not supported by
    any platform.
  • unsharedDataSize {integer} maps to ru_idrss but is not supported by
    any platform.
  • unsharedStackSize {integer} maps to ru_isrss but is not supported by
    any platform.
  • minorPageFault {integer} maps to ru_minflt which is the number of
    minor page faults for the process, see
    this article for more details.
  • majorPageFault {integer} maps to ru_majflt which is the number of
    major page faults for the process, see
    this article for more details. This field is not
    supported on Windows.
  • swappedOut {integer} maps to ru_nswap but is not supported by any
    platform.
  • fsRead {integer} maps to ru_inblock which is the number of times the
    file system had to perform input.
  • fsWrite {integer} maps to ru_oublock which is the number of times the
    file system had to perform output.
  • ipcSent {integer} maps to ru_msgsnd but is not supported by any
    platform.
  • ipcReceived {integer} maps to ru_msgrcv but is not supported by any
    platform.
  • signalsCount {integer} maps to ru_nsignals but is not supported by any
    platform.
  • voluntaryContextSwitches {integer} maps to ru_nvcsw which is the
    number of times a CPU context switch resulted due to a process voluntarily
    giving up the processor before its time slice was completed (usually to
    await availability of a resource). This field is not supported on Windows.
  • involuntaryContextSwitches {integer} maps to ru_nivcsw which is the
    number of times a CPU context switch resulted due to a higher priority
    process becoming runnable or because the current process exceeded its
    time slice. This field is not supported on Windows.

import { resourceUsage } from 'node:process';

console.log(resourceUsage());
/*
  Will output:
  {
    userCPUTime: 82872,
    systemCPUTime: 4143,
    maxRSS: 33164,
    sharedMemorySize: 0,
    unsharedDataSize: 0,
    unsharedStackSize: 0,
    minorPageFault: 2469,
    majorPageFault: 0,
    swappedOut: 0,
    fsRead: 0,
    fsWrite: 8,
    ipcSent: 0,
    ipcReceived: 0,
    signalsCount: 0,
    voluntaryContextSwitches: 79,
    involuntaryContextSwitches: 1
  }
*/

const { resourceUsage } = require('node:process');

console.log(resourceUsage());
/*
  Will output:
  {
    userCPUTime: 82872,
    systemCPUTime: 4143,
    maxRSS: 33164,
    sharedMemorySize: 0,
    unsharedDataSize: 0,
    unsharedStackSize: 0,
    minorPageFault: 2469,
    majorPageFault: 0,
    swappedOut: 0,
    fsRead: 0,
    fsWrite: 8,
    ipcSent: 0,
    ipcReceived: 0,
    signalsCount: 0,
    voluntaryContextSwitches: 79,
    involuntaryContextSwitches: 1
  }
*/

process.send(message[, sendHandle[, options]][, callback])

• message {Object}
• sendHandle {net.Server|net.Socket}
• options {Object} used to parameterize the sending of certain types of
  handles.options supports the following properties:
  • keepOpen {boolean} A value that can be used when passing instances of
    net.Socket. When true, the socket is kept open in the sending process.
    Default: false.
• 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.

The message goes through serialization and parsing. The resulting message might
not be the same as what is originally sent.

process.setegid(id)

• id {string|number} A group name or 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.

import process from 'node:process';

if (process.getegid && process.setegid) {
  console.log(`Current gid: ${process.getegid()}`);
  try {
    process.setegid(501);
    console.log(`New gid: ${process.getegid()}`);
  } catch (err) {
    console.error(`Failed to set gid: ${err}`);
  }
}

const process = require('node:process');

if (process.getegid && process.setegid) {
  console.log(`Current gid: ${process.getegid()}`);
  try {
    process.setegid(501);
    console.log(`New gid: ${process.getegid()}`);
  } catch (err) {
    console.error(`Failed to set gid: ${err}`);
  }
}

This function is only available on POSIX platforms (i.e. not Windows or
Android).
This feature is not available in Worker threads.

process.seteuid(id)

• id {string|number} A user name or 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.

import process from 'node:process';

if (process.geteuid && process.seteuid) {
  console.log(`Current uid: ${process.geteuid()}`);
  try {
    process.seteuid(501);
    console.log(`New uid: ${process.geteuid()}`);
  } catch (err) {
    console.error(`Failed to set uid: ${err}`);
  }
}

const process = require('node:process');

if (process.geteuid && process.seteuid) {
  console.log(`Current uid: ${process.geteuid()}`);
  try {
    process.seteuid(501);
    console.log(`New uid: ${process.geteuid()}`);
  } catch (err) {
    console.error(`Failed to set uid: ${err}`);
  }
}

This function is only available on POSIX platforms (i.e. not Windows or
Android).
This feature is not available in Worker threads.

process.setgid(id)

• id {string|number} The group name or 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.

import process from 'node:process';

if (process.getgid && process.setgid) {
  console.log(`Current gid: ${process.getgid()}`);
  try {
    process.setgid(501);
    console.log(`New gid: ${process.getgid()}`);
  } catch (err) {
    console.error(`Failed to set gid: ${err}`);
  }
}

const process = require('node:process');

if (process.getgid && process.setgid) {
  console.log(`Current gid: ${process.getgid()}`);
  try {
    process.setgid(501);
    console.log(`New gid: ${process.getgid()}`);
  } catch (err) {
    console.error(`Failed to set gid: ${err}`);
  }
}

This function is only available on POSIX platforms (i.e. not Windows or
Android).
This feature is not available in Worker threads.

process.setgroups(groups)

• groups {integer[]}

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.

import process from 'node:process';

if (process.getgroups && process.setgroups) {
  try {
    process.setgroups([501]);
    console.log(process.getgroups()); // new groups
  } catch (err) {
    console.error(`Failed to set groups: ${err}`);
  }
}

const process = require('node:process');

if (process.getgroups && process.setgroups) {
  try {
    process.setgroups([501]);
    console.log(process.getgroups()); // new groups
  } catch (err) {
    console.error(`Failed to set groups: ${err}`);
  }
}

This function is only available on POSIX platforms (i.e. not Windows or
Android).
This feature is not available in Worker threads.

process.setuid(id)

• id {integer | string}

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.

import process from 'node:process';

if (process.getuid && process.setuid) {
  console.log(`Current uid: ${process.getuid()}`);
  try {
    process.setuid(501);
    console.log(`New uid: ${process.getuid()}`);
  } catch (err) {
    console.error(`Failed to set uid: ${err}`);
  }
}

const process = require('node:process');

if (process.getuid && process.setuid) {
  console.log(`Current uid: ${process.getuid()}`);
  try {
    process.setuid(501);
    console.log(`New uid: ${process.getuid()}`);
  } catch (err) {
    console.error(`Failed to set uid: ${err}`);
  }
}

This function is only available on POSIX platforms (i.e. not Windows or
Android).
This feature is not available in Worker threads.

process.setSourceMapsEnabled(val)

Stability: 1 — Experimental

• val {boolean}

This function enables or disables the Source Map v3 support for
stack traces.

It provides same features as launching Node.js process with commandline options
--enable-source-maps.

Only source maps in JavaScript files that are loaded after source maps has been
enabled will be parsed and loaded.

process.setUncaughtExceptionCaptureCallback(fn)

• fn {Function|null}

The process.setUncaughtExceptionCaptureCallback() function sets a function
that will be invoked when an uncaught exception occurs, which will receive the
exception value itself as its first argument.

If such a function is set, the 'uncaughtException' event will
not be emitted. If --abort-on-uncaught-exception was passed from the
command line or set through v8.setFlagsFromString(), the process will
not abort. Actions configured to take place on exceptions such as report
generations will be affected too

To unset the capture function,
process.setUncaughtExceptionCaptureCallback(null) may be used. Calling this
method with a non-null argument while another capture function is set will
throw an error.

Using this function is mutually exclusive with using the deprecated
domain built-in module.

process.stderr

• {Stream}

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.

process.stderr differs from other Node.js streams in important ways. See
note on process I/O for more information.

process.stderr.fd

• {number}

This property refers to the value of underlying file descriptor of
process.stderr. The value is fixed at 2. In Worker threads,
this field does not exist.

process.stdin

• {Stream}

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.