Process | Node.js v8.0.0 Documentation

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# 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'#

Added in: v0.11.12

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'#

Added in: v0.7.7

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'#

Added in: v0.1.7

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'#

Added in: v0.5.10

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'#

Added in: v1.4.1

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'#

Added in: v0.1.18

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}\n`);
    });

    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'#

History

Version | Changes
--- | ---
v7.0.0 | Not handling Promise rejections has been deprecated.
v6.6.0 | Unhandled Promise rejections have been will now emit a process warning.
v1.4.1 | Added in: v1.4.1

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:', p, 'reason:', reason);
      // application specific logging, throwing an error, or other logic here
    });

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

The following will also trigger the `'unhandledRejection'` event to be emitted:

    function SomeResource() {
      // Initially set the loaded status to a rejected promise
      this.loaded = Promise.reject(new Error('Resource not yet loaded!'));
    }

    const resource = new SomeResource();
    // no .catch or .then on resource.loaded for at least a turn

In this example case, it is possible to track the rejection as a developer error as would typically be the case for other `'unhandledRejection'` events. To address such failures, a non-operational `.catch(() => { })` handler may be attached to `resource.loaded`, which would prevent the `'unhandledRejection'` event from being emitted. Alternatively, the `'rejectionHandled'` event may be used.

### Event: 'warning'#

Added in: v6.0.0

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) 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`.

#### 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()#

Added in: v0.7.0

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

## process.arch#

Added in: v0.5.0

- <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#

Added in: v0.1.27

- <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-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#

Added in: 6.4.0

- <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#

Added in: v7.1.0

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)#

Added in: v0.1.17

- `directory` <string>

The `process.chdir()` method changes the current working directory of the Node.js process or throws an exception if doing so fails (for instance, if the specified `directory` does not exist).

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

## process.config#

Added in: v0.7.7

- <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#

Added in: v0.7.2

- <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])#

Added in: v6.1.0

- `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()#

Added in: v0.1.8

- Returns: <string>

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

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

## process.disconnect()#

Added in: v0.7.2

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[, options])#

Added in: 8.0.0

- `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 `process.on('warning')` event.

    // Emit a warning with a code and additional detail.
    process.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 `process.on('warning')` event.

    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])#

Added in: v6.0.0

- `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 `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 type.
    process.emitWarning('Something Happened!', 'CustomWarning');
    // Emits: (node:56338) CustomWarning: Something Happened!

    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 `process.on('warning')` event.

    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 `process.on('warning')` event handler unmodified (and the optional `type`, `code` and `ctor` arguments will be ignored):

    // 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';

    process.emitWarning(myWarning);
    // Emits: (node:56338) [WARN001] CustomWarning: 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 `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, 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#

Added in: v0.1.27

- <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#

Added in: v0.7.7

- <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#

Added in: v0.1.100

- <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])#

Added in: v0.1.13

- `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#

Added in: v0.11.8

- <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()#

Added in: v2.0.0

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()#

Added in: v2.0.0

- 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()#

Added in: v0.1.31

- 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()#

Added in: v0.9.4

- 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()#

Added in: v0.1.28

- 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])#

Added in: v0.7.6

- `time` <Array> The result of a previous call to `process.hrtime()`
- Returns: <Array>

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:

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

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

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

## process.initgroups(user, extra_group)#

Added in: v0.9.4

- `user` <string> | <number> The user name or numeric identifier.
- `extra_group` <string> | <number> A group name or numeric identifier.

The `process.initgroups()` method reads the `/etc/group` file and initializes the group access list, using all groups of which the user is a member. This is a privileged operation that requires that the Node.js process either have `root` access or the `CAP_SETGID` capability.

Note that care must be taken when dropping privileges. Example:

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

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

## process.kill(pid[, signal])#

Added in: v0.0.6

- `pid` <number> A process ID
- `signal` <string> | <number> The signal to send, either as a string or number. Defaults to `'SIGTERM'`.

The `process.kill()` method sends the `signal` to the process identified by `pid`.

Signal names are strings such as `'SIGINT'` or `'SIGHUP'`. See Signal Events and kill(2) for more information.

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

Note: Even though the name of this function is `process.kill()`, it is really just a signal sender, like the `kill` system call. The signal sent may do something other than kill the target process.

For example:

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

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

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

Note: When `SIGUSR1` is received by a Node.js process, Node.js will start the debugger, see Signal Events.

## process.mainModule#

Added in: v0.1.17

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()#

History

Version | Changes
--- | ---
v7.2.0 | Added `external` to the returned object.
v0.1.16 | Added in: v0.1.16

- Returns: <Object>
  - `rss` <integer>
  - `heapTotal` <integer>
  - `heapUsed` <integer>
  - `external` <integer>

The `process.memoryUsage()` method returns an object describing the memory usage of the Node.js process measured in bytes.

For example, the code:

    console.log(process.memoryUsage());

Will generate:

    {
      rss: 4935680,
      heapTotal: 1826816,
      heapUsed: 650472,
      external: 49879
    }

`heapTotal` and `heapUsed` refer to V8's memory usage. `external` refers to the memory usage of C++ objects bound to JavaScript objects managed by V8.

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

History

Version | Changes
--- | ---
v1.8.1 | Additional arguments after `callback` are now supported.
v0.1.26 | Added in: v0.1.26

- `callback` <Function>
- `...args` <any> Additional arguments to pass when invoking the `callback`

The `process.nextTick()` method adds the `callback` to the "next tick queue". Once the current turn of the event loop turn runs to completion, all callbacks currently in the next tick queue will be called.

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

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

This is important when developing APIs in order to give users the opportunity to assign event handlers after an object has been constructed but before any I/O has occurred:

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

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

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

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

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

    // WARNING!  DO NOT USE!  BAD UNSAFE HAZARD!
    function maybeSync(arg, cb) {
      if (arg) {
        cb();
        return;
      }

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

This API is hazardous because in the following case:

    const maybeTrue = Math.random() > 0.5;

    maybeSync(maybeTrue, () => {
      foo();
    });

    bar();

It is not clear whether `foo()` or `bar()` will be called first.

The following approach is much better:

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

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

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

## process.pid#

Added in: v0.1.15

- <integer>

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

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

## process.platform#

Added in: v0.1.16

- <string>

The `process.platform` property returns a string identifying the operating system platform on which the Node.js process is running. For instance `'darwin'`, `'freebsd'`, `'linux'`, `'sunos'` or `'win32'`

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

## process.release#

History

Version | Changes
--- | ---
v4.2.0 | The `lts` property is now supported.
v3.0.0 | Added in: v3.0.0

The `process.release` property returns an Object containing metadata related to the current release, including URLs for the source tarball and headers-only tarball.

`process.release` contains the following properties:

- `name` <string> A value that will always be `'node'` for Node.js. For legacy io.js releases, this will be `'io.js'`.
- `sourceUrl` <string> an absolute URL pointing to a `.tar.gz` file containing the source code of the current release.
- `headersUrl`<string> an absolute URL pointing to a `.tar.gz` file containing only the source header files for the current release. This file is significantly smaller than the full source file and can be used for compiling Node.js native add-ons.
- `libUrl` <string> an absolute URL pointing to 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> a string label identifying the LTS label for this release. If the Node.js release is not an LTS release, this will be `undefined`.

For example:

    {
      name: 'node',
      lts: 'Argon',
      sourceUrl: 'https://nodejs.org/download/release/v4.4.5/node-v4.4.5.tar.gz',
      headersUrl: 'https://nodejs.org/download/release/v4.4.5/node-v4.4.5-headers.tar.gz',
      libUrl: 'https://nodejs.org/download/release/v4.4.5/win-x64/node.lib'
    }

In custom builds from non-release versions of the source tree, only the `name` property may be present. The additional properties should not be relied upon to exist.

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

Added in: v0.5.9

- `message` <Object>
- `sendHandle` <Handle object>
- `options` <Object>
- `callback` <Function>
- Returns: <boolean>

If Node.js is spawned with an IPC channel, the `process.send()` method can be used to send messages to the parent process. Messages will be received as a `'message'` event on the parent's `ChildProcess` object.

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

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

## process.setegid(id)#

Added in: v2.0.0

- `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.

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

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

## process.seteuid(id)#

Added in: v2.0.0

- `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.

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

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

## process.setgid(id)#

Added in: v0.1.31

- `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.

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

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

## process.setgroups(groups)#

Added in: v0.9.4

- `groups` <Array>

The `process.setgroups()` method sets the supplementary group IDs for the Node.js process. This is a privileged operation that requires the Node.js process to have `root` or the `CAP_SETGID` capability.

The `groups` array can contain numeric group IDs, group names or both.

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

## process.setuid(id)#

Added in: v0.1.28

The `process.setuid(id)` method sets the user identity of the process. (See setuid(2).) The `id` can be passed as either a numeric ID or a username string. If a username is specified, the method blocks while resolving the associated numeric ID.

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

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

## process.stderr#

- <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.

Note: `process.stderr` differs from other Node.js streams in important ways, see note on process I/O for more information.

## 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.

For example:

    process.stdin.setEncoding('utf8');

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

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

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

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

## process.stdout#

- <Stream>

The `process.stdout` property returns a stream connected to `stdout` (fd `1`). It is a `net.Socket` (which is a Duplex stream) unless fd `1` refers to a file, in which case it is a Writable stream.

For example, to copy process.stdin to process.stdout:

    process.stdin.pipe(process.stdout);

Note: `process.stdout` differs from other Node.js streams in important ways, see note on process I/O for more information.

### A note on process I/O#

`process.stdout` and `process.stderr` differ from other Node.js streams in important ways:

1. They are used internally by `console.log()` and `console.error()`, respectively.
2. They cannot be closed (`end()` will throw).
3. They will never emit the `'finish'` event.
4. Writes may be synchronous depending on the what the stream is connected to and whether the system is Windows or Unix:
   - Files: synchronous on Windows and Linux
   - TTYs (Terminals): asynchronous on Windows, synchronous on Unix
   - Pipes (and sockets): synchronous on Windows, asynchronous on Unix

These behaviors are partly for historical reasons, as changing them would create backwards incompatibility, but they are also expected by some users.

Synchronous writes avoid problems such as output written with `console.log()` or `console.error()` being unexpectedly interleaved, or not written at all if `process.exit()` is called before an asynchronous write completes. See `process.exit()` for more information.

Warning: Synchronous writes block the event loop until the write has completed. This can be near instantaneous in the case of output to a file, but under high system load, pipes that are not being read at the receiving end, or with slow terminals or file systems, its possible for the event loop to be blocked often enough and long enough to have severe negative performance impacts. This may not be a problem when writing to an interactive terminal session, but consider this particularly careful when doing production logging to the process output streams.

To check if a stream is connected to a TTY context, check the `isTTY` property.

For instance:

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

See the TTY documentation for more information.

## process.title#

Added in: v0.1.104

- <string>

The `process.title` property returns the current process title (i.e. returns the current value of `ps`). Assigning a new value to `process.title` modifies the current value of `ps`.

Note: When a new value is assigned, different platforms will impose different maximum length restrictions on the title. Usually such restrictions are quite limited. For instance, on Linux and macOS, `process.title` is limited to the size of the binary name plus the length of the command line arguments because setting the `process.title` overwrites the `argv` memory of the process. Node.js v0.8 allowed for longer process title strings by also overwriting the `environ` memory but that was potentially insecure and confusing in some (rather obscure) cases.

## process.umask([mask])#

Added in: v0.1.19

- `mask` <number>

The `process.umask()` method sets or returns the Node.js process's file mode creation mask. Child processes inherit the mask from the parent process. The old mask is return if the `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()#

Added in: v0.5.0

- Returns: <number>

The `process.uptime()` method returns the number of seconds the current Node.js process has been running.

Note: The return value includes fractions of a second. Use `Math.floor()` to get whole seconds.

## process.version#

Added in: v0.1.3

- <string>

The `process.version` property returns the Node.js version string.

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

## process.versions#

History

Version | Changes
--- | ---
v4.2.0 | The `icu` property is now supported.
v0.2.0 | Added in: v0.2.0

- <Object>

The `process.versions` property returns an object listing the version strings of Node.js and its dependencies. `process.versions.modules` indicates the current ABI version, which is increased whenever a C++ API changes. Node.js will refuse to load modules that were compiled against a different module ABI version.

    console.log(process.versions);

Will generate an object similar to:

    {
      http_parser: '2.3.0',
      node: '1.1.1',
      v8: '4.1.0.14',
      uv: '1.3.0',
      zlib: '1.2.8',
      ares: '1.10.0-DEV',
      modules: '43',
      icu: '55.1',
      openssl: '1.0.1k',
      unicode: '8.0',
      cldr: '29.0',
      tz: '2016b' }

## 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 `--inspect` and/or `--inspect-brk` options were set, but the port number chosen was invalid or unavailable.
- `>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.