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

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.

Example of listening for exit:

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

Event: 'uncaughtException'

Emitted when an exception bubbles all the way back to the event loop. If a listener is added for this exception, the default action (which is to print a stack trace and exit) will not occur.

Example of listening for uncaughtException:

process.on('uncaughtException', function(err) {
  console.log('Caught exception: ' + err);

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

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

Note that uncaughtException is a very crude mechanism for exception handling and may be removed in the future.

Don't use it, use domains instead. If you do use it, restart your application after every unhandled exception!

Do not use it as the node.js equivalent of On Error Resume Next. An unhandled exception means your application - and by extension node.js itself - is in an undefined state. Blindly resuming means anything could happen.

Think of resuming as pulling the power cord when you are upgrading your system. Nine out of ten times nothing happens - but the 10th time, your system is bust.

You have been warned.

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.on('SIGINT', function() {
  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.


  • 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 behaviour will be removed (node 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 will be unconditionally terminated by Windows about 10 seconds later. On non-Windows platforms, the default behaviour of SIGHUP is to terminate node, but once a listener has been installed its default behaviour 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 on all platforms.
  • SIGSTOP cannot have a listener installed.

Note that Windows does not support sending Signals, but node offers some emulation with process.kill(), and child_process.kill():

  • Sending signal 0 can be used to search for the existence of a process
  • Sending SIGINT, SIGTERM, and SIGKILL cause the unconditional exit of the target process.


A Writable Stream to stdout.

Example: the definition of console.log

console.log = function(d) {
  process.stdout.write(d + '\n');

process.stderr and process.stdout are unlike other streams in Node in that writes to them are usually blocking.

  • They are blocking in the case that they refer to regular files or TTY file descriptors.
  • In the case they refer to pipes:
    • They are blocking in Linux/Unix.
    • They are non-blocking like other streams in Windows.

To check if Node 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)"
$ echo "foo" | node -p "Boolean(process.stdin.isTTY)"

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

See the tty docs for more information.


A writable stream to stderr.

process.stderr and process.stdout are unlike other streams in Node in that writes to them are usually blocking.

  • They are blocking in the case that they refer to regular files or TTY file descriptors.
  • In the case they refer to pipes:
    • They are blocking in Linux/Unix.
    • They are non-blocking like other streams in Windows.


A Readable Stream for stdin.

Example of opening standard input and listening for both events:


process.stdin.on('readable', function(chunk) {
  var chunk =;
  if (chunk !== null) {
    process.stdout.write('data: ' + chunk);

process.stdin.on('end', function() {

As a Stream, process.stdin can also be used in "old" mode that is compatible with scripts written for node prior 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.


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(function(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


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




This is the set of node-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 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']


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


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

console.log('Starting directory: ' + process.cwd());
try {
  console.log('New directory: ' + process.cwd());
catch (err) {
  console.log('chdir: ' + err);


Returns the current working directory of the process.

console.log('Current directory: ' + process.cwd());


An object containing the user environment. See environ(7).


Ends the process with the specified code. If omitted, exit uses the 'success' code 0.

To exit with a 'failure' code:


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


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

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());


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

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

if (process.getgid && process.setgid) {
  console.log('Current gid: ' + process.getgid());
  try {
    console.log('New gid: ' + process.getgid());
  catch (err) {
    console.log('Failed to set gid: ' + err);


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

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());


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

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 {
    console.log('New uid: ' + process.getuid());
  catch (err) {
    console.log('Failed to set uid: ' + err);


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

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)

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.initgroups(user, extra_group)

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

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 ]


A compiled-in property that exposes NODE_VERSION.

console.log('Version: ' + process.version);


A property exposing version strings of node and its dependencies.


Will print something like:

{ http_parser: '1.0',
  node: '0.10.4',
  v8: '',
  ares: '1.9.0-DEV',
  uv: '0.10.3',
  zlib: '1.2.3',
  modules: '11',
  openssl: '1.0.1e' }


An Object containing the JavaScript representation of the configure options that were used to compile the current node 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: [] },
   { host_arch: 'x64',
     node_install_npm: 'true',
     node_prefix: '',
     node_shared_cares: 'false',
     node_shared_http_parser: 'false',
     node_shared_libuv: 'false',
     node_shared_v8: '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' } }

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.

Note that just because 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', function() {
  console.log('Got SIGHUP signal.');

setTimeout(function() {
}, 100);

process.kill(, 'SIGHUP');

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

The PID of the process.

console.log('This process is pid ' +;


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.


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

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


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

console.log('This platform is ' + process.platform);


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

var util = require('util');


This will generate:

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

heapTotal and heapUsed refer to V8's memory usage.


On the next loop around the event loop call this callback. This is not a simple alias to setTimeout(fn, 0), it's much more efficient. It typically runs before any other I/O events fire, but there are some exceptions. See process.maxTickDepth below.

process.nextTick(function() {
  console.log('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) {

  process.nextTick(function() {

var thing = new MyThing();

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

It is very important for APIs to be either 100% synchronous or 100% asynchronous. Consider this example:

function maybeSync(arg, cb) {
  if (arg) {

  fs.stat('file', cb);

This API is hazardous. If you do this:

maybeSync(true, function() {

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

This approach is much better:

function definitelyAsync(arg, cb) {
  if (arg) {

  fs.stat('file', cb);


  • {Number} Default = 1000

Callbacks passed to process.nextTick will usually be called at the end of the current flow of execution, and are thus approximately as fast as calling a function synchronously. Left unchecked, this would starve the event loop, preventing any I/O from occurring.

Consider this code:

process.nextTick(function foo() {

In order to avoid the situation where Node is blocked by an infinite loop of recursive series of nextTick calls, it defers to allow some I/O to be done every so often.

The process.maxTickDepth value is the maximum depth of nextTick-calling nextTick-callbacks that will be evaluated before allowing other forms of I/O to occur.


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.

var oldmask, newmask = 0644;

oldmask = process.umask(newmask);
console.log('Changed umask from: ' + oldmask.toString(8) +
            ' to ' + newmask.toString(8));


Number of seconds Node has been running.


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

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