nodejs/doc/api/process.md

process

The process object is a global object and can be accessed from anywhere. It is an instance of EventEmitter.

Event: 'beforeExit'

This event is emitted when Node.js empties its event loop and has nothing else to schedule. Normally, Node.js exits when there is no work scheduled, but a listener for 'beforeExit' can make asynchronous calls, and cause Node.js to continue.

'beforeExit' is not emitted for conditions causing explicit termination, such as process.exit() or uncaught exceptions, and should not be used as an alternative to the 'exit' event unless the intention is to schedule more work.

Event: 'disconnect'

If process is spawned with an IPC channel, 'disconnect' will be emitted when IPC channel is closed. Read more in child_process 'disconnect' event doc.

Event: 'exit'

Emitted when the process is about to exit. There is no way to prevent the exiting of the event loop at this point, and once all 'exit' listeners have finished running the process will exit. Therefore you must only perform synchronous operations in this handler. This is a good hook to perform checks on the module's state (like for unit tests). The callback takes one argument, the code the process is exiting with.

This event is only emitted when Node.js exits explicitly by process.exit() or implicitly by the event loop draining.

Example of listening for 'exit':

process.on('exit', (code) => {
  // do *NOT* do this
  setTimeout(() => {
    console.log('This will not run');
  }, 0);
  console.log('About to exit with code:', code);
});

Event: 'message'

• message {Object} a parsed JSON object or primitive value
• sendHandle {Handle object} a net.Socket or net.Server object, or undefined.

Messages sent by ChildProcess.send() are obtained using the 'message' event on the child's process object.

Event: 'rejectionHandled'

Emitted whenever a Promise was rejected and an error handler was attached to it (for example with promise.catch()) later than after an event loop turn. This event is emitted with the following arguments:

• p the promise that was previously emitted in an 'unhandledRejection' event, but which has now 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 is a growing-and-shrinking list of unhandled rejections. In synchronous code, the 'uncaughtException' event tells you when the list of unhandled exceptions grows. And in asynchronous code, the 'unhandledRejection' event tells you when the list of unhandled rejections grows, while the 'rejectionHandled' event tells you when the list of unhandled rejections shrinks.

For example using the rejection detection hooks in order to keep a map of all the rejected promise reasons at a given time:

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

This map will grow and shrink over time, reflecting rejections that start unhandled and then become handled. You could record the errors in some error log, either periodically (probably best for long-running programs, allowing you to clear the map, which in the case of a very buggy program could grow indefinitely) or upon process exit (more convenient for scripts).

Event: 'uncaughtException'

The 'uncaughtException' event is emitted when an 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.

For example:

process.on('uncaughtException', (err) => {
  console.log(`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'.

Event: 'unhandledRejection'

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. Such promises can be caught and handled using promise.catch() and rejections are propagated through a promise chain. This event is useful for detecting and keeping track of promises that were rejected whose rejections were not handled yet. This event is emitted with the following arguments:

• reason the object with which the promise was rejected (usually an Error instance).
• p the promise that was rejected.

Here is an example that logs every unhandled rejection to the console

process.on('unhandledRejection', (reason, p) => {
    console.log("Unhandled Rejection at: Promise ", p, " reason: ", reason);
    // application specific logging, throwing an error, or other logic here
});

For example, here is a rejection that will trigger the 'unhandledRejection' event:

somePromise.then((res) => {
  return reportToUser(JSON.pasre(res)); // note the typo (`pasre`)
}); // no `.catch` or `.then`

Here is an example of a coding pattern that will also trigger 'unhandledRejection':

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 cases like this, you may not want to track the rejection as a developer error like you would for other 'unhandledRejection' events. To address this, you can either attach a dummy .catch(() => { }) handler to resource.loaded, preventing the 'unhandledRejection' event from being emitted, or you can use the 'rejectionHandled' event.

Event: 'warning'

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 event handler for 'warning' events is called with a single warning argument whose value is an Error object. There are three key properties that describe the warning:

• name - The name of the warning (currently Warning by default).
• message - A system-provided description of the warning.
• stack - 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
> event.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!'));
> event.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> Do not do that!

The --trace-warnings command-line option can be used to have the default console output for warnings include the full stack trace of the warning.

Emitting custom warnings

The process.emitWarning() method can be used to issue custom or application specific warnings.

// Emit a warning using a string...
process.emitWarning('Something happened!');
  // Prints: (node 12345) Warning: Something happened!

// Emit a warning using an object...
process.emitWarning('Something Happened!', 'CustomWarning');
  // Prints: (node 12345) CustomWarning: Something happened!

// Emit a warning using a custom Error object...
class CustomWarning extends Error {
  constructor(message) {
    super(message);
    this.name = 'CustomWarning';
    Error.captureStackTrace(this, CustomWarning);
  }
}
const myWarning = new CustomWarning('Something happened!');
process.emitWarning(myWarning);
  // Prints: (node 12345) CustomWarning: Something happened!

Emitting custom deprecation warnings

Custom deprecation warnings can be emitted by setting the name of a custom warning to DeprecationWarning. For instance:

process.emitWarning('This API is deprecated', 'DeprecationWarning');

Or,

const err = new Error('This API is deprecated');
err.name = 'DeprecationWarning';
process.emitWarning(err);

Launching Node.js using the --throw-deprecation command line flag will cause custom deprecation warnings to be thrown as exceptions.

Using the --trace-deprecation command line flag will cause the custom deprecation to be printed to stderr along with the stack trace.

Using the --no-deprecation command line flag will suppress all reporting of the custom deprecation.

The *-deprecation command line flags only affect warnings that use the name DeprecationWarning.

Exit Codes

Node.js will normally exit with a 0 status code when no more async operations are pending. The following status codes are used in other cases:

• 1 Uncaught Fatal Exception - There was an uncaught exception, and it was not handled by a domain or an 'uncaughtException' event handler.
• 2 - Unused (reserved by Bash for builtin misuse)
• 3 Internal JavaScript Parse Error - The JavaScript source code internal in Node.js's bootstrapping process caused a parse error. This is extremely rare, and generally can only happen during development of Node.js itself.
• 4 Internal JavaScript Evaluation Failure - The JavaScript source code internal in Node.js's bootstrapping process failed to return a function value when evaluated. This is extremely rare, and generally can only happen during development of Node.js itself.
• 5 Fatal Error - There was a fatal unrecoverable error in V8. Typically a message will be printed to stderr with the prefix FATAL ERROR.
• 6 Non-function Internal Exception Handler - There was an uncaught exception, but the internal fatal exception handler function was somehow set to a non-function, and could not be called.
• 7 Internal Exception Handler Run-Time Failure - There was an uncaught exception, and the internal fatal exception handler function itself threw an error while attempting to handle it. This can happen, for example, if a 'uncaughtException' or domain.on('error') handler throws an error.
• 8 - Unused. In previous versions of Node.js, exit code 8 sometimes indicated an uncaught exception.
• 9 - Invalid Argument - Either an unknown option was specified, or an option requiring a value was provided without a value.
• 10 Internal JavaScript Run-Time Failure - The JavaScript source code internal in Node.js's bootstrapping process threw an error when the bootstrapping function was called. This is extremely rare, and generally can only happen during development of Node.js itself.
• 12 Invalid Debug Argument - The --debug and/or --debug-brk options were set, but an invalid port number was chosen.
• >128 Signal Exits - If Node.js receives a fatal signal such as SIGKILL or SIGHUP, then its exit code will be 128 plus the value of the signal code. This is a standard Unix practice, since exit codes are defined to be 7-bit integers, and signal exits set the high-order bit, and then contain the value of the signal code.

Signal Events

Emitted when the processes receives a signal. See sigaction(2) for a list of standard POSIX signal names such as SIGINT, SIGHUP, etc.

Example of listening for SIGINT:

// Start reading from stdin so we don't exit.
process.stdin.resume();

process.on('SIGINT', () => {
  console.log('Got SIGINT.  Press Control-D to exit.');
});

An easy way to send the SIGINT signal is with Control-C in most terminal programs.

Note:

• SIGUSR1 is reserved by Node.js to start the debugger. It's possible to install a listener but that won't 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.

Note that 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()

This causes Node.js to emit an abort. This will cause Node.js to exit and generate a core file.

process.arch

What processor architecture you're running on: 'arm', 'ia32', or 'x64'.

console.log('This processor architecture is ' + process.arch);

process.argv

An array containing the command line arguments. The first element will be 'node', the second element will be the name of the JavaScript file. The next elements will be any additional command line arguments.

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

This will generate:

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

process.chdir(directory)

Changes the current working directory of the process or throws an exception if that fails.

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

process.config

An Object containing the JavaScript representation of the configure options that were 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} Set to false after process.disconnect() is called

If process.connected is false, it is no longer possible to send messages.

process.cpuUsage([previousValue])

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 the current working directory of the process.

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

process.disconnect()

Close the IPC channel to the parent process, allowing this child to exit gracefully once there are no other connections keeping it alive.

Identical to the parent process's ChildProcess.disconnect().

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

process.env

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' }

You can write to this object, but changes won't be reflected outside of your process. That means that the following won't work:

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

But this 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

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:

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

process.execArgv

This is the set of Node.js-specific command line options from the executable that started the process. These options do not show up in process.argv, 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.

Example:

$ node --harmony script.js --version

results in process.execArgv:

['--harmony']

and process.argv:

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

process.execPath

This is the absolute pathname of the executable that started the process.

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 as quickly as possible with the specified exit code. If the code is omitted, exit uses either the 'success' code 0 or the value of process.exitCode if specified.

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 it's 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 non-blocking and may occur over multiple ticks of the Node.js event loop. Calling process.exit(), however, forces the process to exit before those additional writes to stdout can be performed.

Rather than calling process.exit() directly, the code should set the process.exitCode and allow the process to exit naturally by avoiding scheduling any additional work for the event loop:

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

If it is necessary to terminate the Node.js process due to an error condition, throwing an uncaught error and allowing the process to terminate accordingly is safer than calling process.exit().

process.exitCode

A number which will be the process exit code, when the process either exits gracefully, or is exited via process.exit() without specifying a code.

Specifying a code to process.exit(code) will override any previous setting of process.exitCode.

process.getegid()

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Gets the effective group identity of the process. (See getegid(2).) This is the numerical group id, not the group name.

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

process.geteuid()

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Gets the effective user identity of the process. (See geteuid(2).) This is the numerical userid, not the username.

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

process.getgid()

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Gets the group identity of the process. (See getgid(2).) This is the numerical group id, not the group name.

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

process.getgroups()

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

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.

process.getuid()

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Gets the user identity of the process. (See getuid(2).) This is the numerical userid, not the username.

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

process.hrtime()

Returns the current high-resolution real time in a [seconds, nanoseconds] tuple Array. It is relative to an arbitrary time in the past. It is not related to the time of day and therefore not subject to clock drift. The primary use is for measuring performance between intervals.

You may pass in the result of a previous call to process.hrtime() to get a diff reading, useful for benchmarks and measuring intervals:

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

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

  console.log('benchmark took %d nanoseconds', diff[0] * 1e9 + diff[1]);
  // benchmark took 1000000527 nanoseconds
}, 1000);

process.initgroups(user, extra_group)

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Reads /etc/group and initializes the group access list, using all groups of which the user is a member. This is a privileged operation, meaning you need to be root or have the CAP_SETGID capability.

user is a user name or user ID. extra_group is a group name or group ID.

Some care needs to be taken when dropping privileges. Example:

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

process.kill(pid[, signal])

Send a signal to a process. pid is the process id and signal is the string describing the signal to send. Signal names are strings like 'SIGINT' or 'SIGHUP'. If omitted, the signal will be 'SIGTERM'. See Signal Events and kill(2) for more information.

Will throw an error if target does not exist, and as a special case, a signal of 0 can be used to test for the existence of a process. Windows platforms will throw an error if the pid is used to kill a process group.

Note that 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.

Example of sending a signal to yourself:

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

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

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

Note: When SIGUSR1 is received by Node.js it starts the debugger, see Signal Events.

process.mainModule

Alternate way to retrieve require.main. The difference is that if the main module changes at runtime, require.main might 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, it will be undefined if there was no entry script.

process.memoryUsage()

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

const util = require('util');

console.log(util.inspect(process.memoryUsage()));

This will generate:

{ rss: 4935680,
  heapTotal: 1826816,
  heapUsed: 650472 }

heapTotal and heapUsed refer to V8's memory usage.

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

• callback {Function}

Once the current event loop turn runs to completion, call the callback function.

This is not a simple alias to setTimeout(fn, 0), it's much more efficient. It runs before any additional I/O events (including timers) fire in subsequent ticks of the event loop.

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

This is important in developing APIs where you want to give the user the chance to assign event handlers after an object has been constructed, but before any I/O has occurred.

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

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

var 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. If you do this:

maybeSync(true, () => {
  foo();
});
bar();

then it's not clear whether foo() or bar() will be called first.

This approach is much better:

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

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

Note: the nextTick queue is completely drained on each pass of the event loop before additional I/O is processed. As a result, recursively setting nextTick callbacks will block any I/O from happening, just like a while(true); loop.

process.pid

The PID of the process.

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

process.platform

What platform you're running on: 'darwin', 'freebsd', 'linux', 'sunos' or 'win32'

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

process.release

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: a string with a value that will always be 'node' for Node.js. For legacy io.js releases, this will be 'io.js'.
• sourceUrl: a complete URL pointing to a .tar.gz file containing the source of the current release.
• headersUrl: a complete URL pointing to a .tar.gz file containing only the header files for the current release. This file is significantly smaller than the full source file and can be used for compiling add-ons against Node.js.
• libUrl: a complete URL pointing to an node.lib file matching the architecture and version of the current release. This file is used for compiling add-ons against Node.js. This property is only present on Windows builds of Node.js and will be missing on all other platforms.

e.g.

{ name: 'node',
  sourceUrl: 'https://nodejs.org/download/release/v4.0.0/node-v4.0.0.tar.gz',
  headersUrl: 'https://nodejs.org/download/release/v4.0.0/node-v4.0.0-headers.tar.gz',
  libUrl: 'https://nodejs.org/download/release/v4.0.0/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.send(message[, sendHandle[, options]][, callback])

• message {Object}
• sendHandle {Handle object}
• options {Object}
• callback {Function}
• Return: {Boolean}

When Node.js is spawned with an IPC channel attached, it can send messages to its parent process using process.send(). Each will be received as a 'message' event on the parent's ChildProcess object.

Note: this function uses JSON.stringify() internally to serialize the message.

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

process.setegid(id)

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Sets the effective group identity of the process. (See setegid(2).) This accepts either a numerical ID or a group name string. If a group name is specified, this method blocks while resolving it to a numerical ID.

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

process.seteuid(id)

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Sets the effective user identity of the process. (See seteuid(2).) This accepts either a numerical ID or a username string. If a username is specified, this method blocks while resolving it to a numerical ID.

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

process.setgid(id)

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Sets the group identity of the process. (See setgid(2).) This accepts either a numerical ID or a group name string. If a group name is specified, this method blocks while resolving it to a numerical ID.

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

process.setgroups(groups)

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Sets the supplementary group IDs. This is a privileged operation, meaning you need to be root or have the CAP_SETGID capability.

The list can contain group IDs, group names or both.

process.setuid(id)

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Sets the user identity of the process. (See setuid(2).) This accepts either a numerical ID or a username string. If a username is specified, this method blocks while resolving it to a numerical ID.

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

process.stderr

A writable stream to stderr (on fd 2).

process.stderr and process.stdout are unlike other streams in Node.js in that they cannot be closed (end() will throw), they never emit the 'finish' event and that writes can block when output is redirected to a file (although disks are fast and operating systems normally employ write-back caching so it should be a very rare occurrence indeed.)

process.stdin

A Readable Stream for stdin (on fd 0).

Example of opening standard input and listening for both events:

process.stdin.setEncoding('utf8');

process.stdin.on('readable', () => {
  var chunk = process.stdin.read();
  if (chunk !== null) {
    process.stdout.write(`data: ${chunk}`);
  }
});

process.stdin.on('end', () => {
  process.stdout.write('end');
});

As a Stream, process.stdin can also be used in "old" mode that is compatible with scripts written for node.js prior to v0.10. For more information see Stream compatibility.

In "old" Streams mode the stdin stream is paused by default, so one must call process.stdin.resume() to read from it. Note also that calling process.stdin.resume() itself would switch stream to "old" mode.

If you are starting a new project you should prefer a more recent "new" Streams mode over "old" one.

process.stdout

A Writable Stream to stdout (on fd 1).

For example, a console.log equivalent could look like this:

console.log = (msg) => {
  process.stdout.write(`${msg}\n`);
};

process.stderr and process.stdout are unlike other streams in Node.js in that they cannot be closed (end() will throw), they never emit the 'finish' event and that writes can block when output is redirected to a file (although disks are fast and operating systems normally employ write-back caching so it should be a very rare occurrence indeed.)

To check if Node.js is being run in a TTY context, read the isTTY property on process.stderr, process.stdout, or process.stdin:

$ node -p "Boolean(process.stdin.isTTY)"
true
$ echo "foo" | node -p "Boolean(process.stdin.isTTY)"
false

$ node -p "Boolean(process.stdout.isTTY)"
true
$ node -p "Boolean(process.stdout.isTTY)" | cat
false

See the tty docs for more information.

process.title

Getter/setter to set what is displayed in ps.

When used as a setter, the maximum length is platform-specific and probably short.

On Linux and OS X, it's limited to the size of the binary name plus the length of the command line arguments because it overwrites the argv memory.

v0.8 allowed for longer process title strings by also overwriting the environ memory but that was potentially insecure/confusing in some (rather obscure) cases.

process.umask([mask])

Sets or reads the process's file mode creation mask. Child processes inherit the mask from the parent process. Returns the old mask if mask argument is given, otherwise returns the current mask.

const newmask = 0o022;
const oldmask = process.umask(newmask);
console.log(
  `Changed umask from ${oldmask.toString(8)} to ${newmask.toString(8)}`
);

process.uptime()

Number of seconds Node.js has been running.

process.version

A compiled-in property that exposes NODE_VERSION.

console.log(`Version: ${process.version}`);

process.versions

A property exposing version strings of Node.js and its dependencies.

console.log(process.versions);

Will print something like:

{ http_parser: '2.3.0',
  node: '1.1.1',
  v8: '4.1.0.14',
  uv: '1.3.0',
  zlib: '1.2.8',
  ares: '1.10.0-DEV',
  modules: '43',
  icu: '55.1',
  openssl: '1.0.1k' }