Table of Contents

• Process
  • Process Events
    • Event: 'beforeExit'
    • Event: 'disconnect'
    • Event: 'exit'
    • Event: 'message'
    • Event: 'rejectionHandled'
    • Event: 'uncaughtException'
      • Warning: Using 'uncaughtException' correctly
    • Event: 'unhandledRejection'
    • Event: 'warning'
      • Emitting custom warnings
    • Signal Events
  • process.abort()
  • process.arch
  • process.argv
  • process.argv0
  • process.channel
  • process.chdir(directory)
  • process.config
  • process.connected
  • process.cpuUsage([previousValue])
  • process.cwd()
  • process.disconnect()
  • process.emitWarning(warning[, name][, ctor])
    • Avoiding duplicate warnings
  • process.env
  • 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.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
    • A note on process I/O
  • 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 value
• sendHandle <Handle object> a net.Socket or net.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 an Error object).
• p the Promise 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!'));
}

var 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 (currently Warning 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.

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.

Emitting custom warnings#

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

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 and SIGINT have default handlers on non-Windows platforms that resets 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.
• 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 attempt to call JS listeners. Doing so might lead to the process hanging in an endless loop, since listeners attached using process.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 ChildProcess.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#

• <string>

The process.arch property returns a String identifying the processor architecture that the Node.js process is currently running on. For instance 'arm', 'ia32', or 'x64'.

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

process.argv#

• <Array>

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#

• <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#

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.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.log(`chdir: ${err}`);
}

process.config#

• <Object>

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#

• <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.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.

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.emitWarning(warning[, name][, ctor])#

• warning <string> | <Error> The warning to emit.
• name <string> When warning is a String, name is the name to use for the warning. Default: Warning.
• 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 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 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, it is recommended to place the emitWarning() behind a simple boolean flag as illustrated in the example below:

function emitMyWarning() {
  if (!emitMyWarning.warned) {
    emitMyWarning.warned = true;
    process.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. 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.execArgv#

• <Object>

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#

• <string>

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 to 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:

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#

• <integer>

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:

var time = process.hrtime();
// [ 1800216, 25 ]

setTimeout(() => {
  var 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.