Table of Contents

• Process
  • Event: 'beforeExit'
  • Event: 'exit'
  • Event: 'message'
  • Event: 'rejectionHandled'
  • Event: 'uncaughtException'
    • Warning: Using 'uncaughtException' correctly
  • Event: 'unhandledRejection'
  • Exit Codes
  • Signal Events
  • process.abort()
  • process.arch
  • process.argv
  • process.chdir(directory)
  • process.config
  • process.connected
  • process.cpuUsage([previousValue])
  • process.cwd()
  • process.disconnect()
  • process.env
  • process.execArgv
  • process.execPath
  • process.exit([code])
  • process.exitCode
  • process.getegid()
  • process.geteuid()
  • process.getgid()
  • process.getgroups()
  • process.getuid()
  • process.hrtime()
  • process.initgroups(user, extra_group)
  • process.kill(pid[, signal])
  • process.mainModule
  • process.memoryUsage()
  • process.nextTick(callback[, arg][, ...])
  • process.pid
  • process.platform
  • process.release
  • process.send(message[, sendHandle][, callback])
  • process.setegid(id)
  • process.seteuid(id)
  • process.setgid(id)
  • process.setgroups(groups)
  • process.setuid(id)
  • process.stderr
  • process.stdin
  • process.stdout
  • process.title
  • process.umask([mask])
  • process.uptime()
  • process.version
  • process.versions

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: '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 .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.

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 process.on('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(7) 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 child_process.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#