nodejs/doc/api/cli.md

# Command Line Options

<!--introduced_in=v5.9.1-->
<!--type=misc-->

Node.js comes with a variety of CLI options. These options expose built-in
debugging, multiple ways to execute scripts, and other helpful runtime options.

To view this documentation as a manual page in a terminal, run `man node`.

## Synopsis

`node [options] [V8 options] [script.js | -e "script" | -] [--] [arguments]`

`node inspect [script.js | -e "script" | <host>:<port>] …`

`node --v8-options`

Execute without arguments to start the [REPL][].

_For more info about `node inspect`, please see the [debugger][] documentation._

## Options
<!-- YAML
changes:
  - version: v10.12.0
    pr-url: https://github.com/nodejs/node/pull/23020
    description: Underscores instead of dashes are now allowed for
                 Node.js options as well, in addition to V8 options.
-->

All options, including V8 options, allow words to be separated by both
dashes (`-`) or underscores (`_`).

For example, `--pending-deprecation` is equivalent to `--pending_deprecation`.

### `-`
<!-- YAML
added: v8.0.0
-->

Alias for stdin, analogous to the use of - in other command line utilities,
meaning that the script will be read from stdin, and the rest of the options
are passed to that script.

### `--`
<!-- YAML
added: v6.11.0
-->

Indicate the end of node options. Pass the rest of the arguments to the script.
If no script filename or eval/print script is supplied prior to this, then
the next argument will be used as a script filename.

### `--abort-on-uncaught-exception`
<!-- YAML
added: v0.10
-->

Aborting instead of exiting causes a core file to be generated for post-mortem
analysis using a debugger (such as `lldb`, `gdb`, and `mdb`).

If this flag is passed, the behavior can still be set to not abort through
[`process.setUncaughtExceptionCaptureCallback()`][] (and through usage of the
`domain` module that uses it).

### `--completion-bash`
<!-- YAML
added: v10.12.0
-->

Print source-able bash completion script for Node.js.
```console
$ node --completion-bash > node_bash_completion
$ source node_bash_completion
```

### `--diagnostic-report-directory=directory`
<!-- YAML
added: v11.8.0
-->

Location at which the report will be generated.

### `--diagnostic-report-filename=filename`
<!-- YAML
added: v11.8.0
-->

Name of the file to which the report will be written.

### `--diagnostic-report-on-fatalerror`
<!-- YAML
added: v11.8.0
-->

Enables the report to be triggered on fatal errors (internal errors within
the Node.js runtime such as out of memory) that lead to termination of the
application, if `--experimental-report` is enabled. Useful to inspect various
diagnostic data elements such as heap, stack, event loop state, resource
consumption etc. to reason about the fatal error.

### `--diagnostic-report-on-signal`
<!-- YAML
added: v11.8.0
-->

Enables report to be generated upon receiving the specified (or predefined)
signal to the running Node.js process, if `--experimental-report` is enabled.
The signal to trigger the report is specified through `--diagnostic-report-signal`.

### `--diagnostic-report-signal=signal`
<!-- YAML
added: v11.8.0
-->

Sets or resets the signal for report generation (not supported on Windows).
Default signal is `SIGUSR2`.

### `--diagnostic-report-uncaught-exception`
<!-- YAML
added: v11.8.0
-->

Enables report to be generated on un-caught exceptions, if
`--experimental-report` is enabled. Useful when inspecting JavaScript stack in
conjunction with native stack and other runtime environment data.

### `--enable-fips`
<!-- YAML
added: v6.0.0
-->

Enable FIPS-compliant crypto at startup. (Requires Node.js to be built with
`./configure --openssl-fips`.)

### `--entry-type=type`
<!-- YAML
added: REPLACEME
-->

Used with `--experimental-modules`, this configures Node.js to interpret the
initial entry point as CommonJS or as an ES module.

Valid values are `"commonjs"` and `"module"`. The default is to infer from
the file extension and the `"type"` field in the nearest parent `package.json`.

Works for executing a file as well as `--eval`, `--print`, `STDIN`.

### `--es-module-specifier-resolution=mode`
<!-- YAML
added: REPLACEME
-->

To be used in conjunction with `--experimental-modules`. Sets the resolution
algorithm for resolving specifiers. Valid options are `explicit` and `node`.

The default is `explicit`, which requires providing the full path to a
module. The `node` mode will enable support for optional file extensions and
the ability to import a directory that has an index file.

Please see [customizing esm specifier resolution][] for example usage.

### `--experimental-json-modules`
<!-- YAML
added: REPLACEME
-->

Enable experimental JSON support for the ES Module loader.

### `--experimental-modules`
<!-- YAML
added: v8.5.0
-->

Enable experimental ES module support and caching modules.

### `--experimental-policy`
<!-- YAML
added: v11.8.0
-->

Use the specified file as a security policy.

### `--experimental-repl-await`
<!-- YAML
added: v10.0.0
-->

Enable experimental top-level `await` keyword support in REPL.

### `--experimental-report`
<!-- YAML
added: v11.8.0
-->

Enable experimental diagnostic report feature.

### `--experimental-vm-modules`
<!-- YAML
added: v9.6.0
-->

Enable experimental ES Module support in the `vm` module.

### `--force-fips`
<!-- YAML
added: v6.0.0
-->

Force FIPS-compliant crypto on startup. (Cannot be disabled from script code.)
(Same requirements as `--enable-fips`.)

### `--frozen-intrinsics`
<!-- YAML
added: v11.12.0
-->

> Stability: 1 - Experimental

Enable experimental frozen intrinsics like `Array` and `Object`.

Support is currently only provided for the root context and no guarantees are
currently provided that `global.Array` is indeed the default intrinsic
reference.

**Code breakage is highly likely with this flag**, especially since limited
support for subclassing builtins is provided currently due to ECMA-262 bug
https://github.com/tc39/ecma262/pull/1320.

Both of the above may change in future updates, which will be breaking changes.

### `--heapsnapshot-signal=signal`
<!-- YAML
added: REPLACEME
-->

Generates a heap snapshot each time the process receives the specified signal.
`signal` must be a valid signal name. Disabled by default.

### `--http-parser=library`
<!-- YAML
added: v11.4.0
-->

Chooses an HTTP parser library. Available values are:

- `llhttp` for https://llhttp.org/
- `legacy` for https://github.com/nodejs/http-parser

The default is `llhttp`, unless otherwise specified when building Node.js.

This flag exists to aid in experimentation with the internal implementation of
the Node.js http parser.
This flag is likely to become a no-op and removed at some point in the future.

### `--icu-data-dir=file`
<!-- YAML
added: v0.11.15
-->

Specify ICU data load path. (Overrides `NODE_ICU_DATA`.)

### `--inspect-brk[=[host:]port]`
<!-- YAML
added: v7.6.0
-->

Activate inspector on `host:port` and break at start of user script.
Default `host:port` is `127.0.0.1:9229`.

### `--inspect-port=[host:]port`
<!-- YAML
added: v7.6.0
-->

Set the `host:port` to be used when the inspector is activated.
Useful when activating the inspector by sending the `SIGUSR1` signal.

Default host is `127.0.0.1`.

See the [security warning](#inspector_security) below regarding the `host`
parameter usage.

### `--inspect[=[host:]port]`
<!-- YAML
added: v6.3.0
-->

Activate inspector on `host:port`. Default is `127.0.0.1:9229`.

V8 inspector integration allows tools such as Chrome DevTools and IDEs to debug
and profile Node.js instances. The tools attach to Node.js instances via a
tcp port and communicate using the [Chrome DevTools Protocol][].

<a id="inspector_security"></a>
#### Warning: binding inspector to a public IP:port combination is insecure

Binding the inspector to a public IP (including `0.0.0.0`) with an open port is
insecure, as it allows external hosts to connect to the inspector and perform
a [remote code execution][] attack.

If specifying a host, make sure that either:

* The host is not accessible from public networks.
* A firewall disallows unwanted connections on the port.

**More specifically, `--inspect=0.0.0.0` is insecure if the port (`9229` by
default) is not firewall-protected.**

See the [debugging security implications][] section for more information.

### `--loader=file`
<!-- YAML
added: v9.0.0
-->

Specify the `file` of the custom [experimental ECMAScript Module][] loader.

### `--max-http-header-size=size`
<!-- YAML
added: v11.6.0
-->

Specify the maximum size, in bytes, of HTTP headers. Defaults to 8KB.

### `--napi-modules`
<!-- YAML
added: v7.10.0
-->

This option is a no-op. It is kept for compatibility.

### `--no-deprecation`
<!-- YAML
added: v0.8.0
-->

Silence deprecation warnings.

### `--no-force-async-hooks-checks`
<!-- YAML
added: v9.0.0
-->

Disables runtime checks for `async_hooks`. These will still be enabled
dynamically when `async_hooks` is enabled.

### `--no-warnings`
<!-- YAML
added: v6.0.0
-->

Silence all process warnings (including deprecations).

### `--openssl-config=file`
<!-- YAML
added: v6.9.0
-->

Load an OpenSSL configuration file on startup. Among other uses, this can be
used to enable FIPS-compliant crypto if Node.js is built with
`./configure --openssl-fips`.

### `--pending-deprecation`
<!-- YAML
added: v8.0.0
-->

Emit pending deprecation warnings.

Pending deprecations are generally identical to a runtime deprecation with the
notable exception that they are turned *off* by default and will not be emitted
unless either the `--pending-deprecation` command line flag, or the
`NODE_PENDING_DEPRECATION=1` environment variable, is set. Pending deprecations
are used to provide a kind of selective "early warning" mechanism that
developers may leverage to detect deprecated API usage.

### `--preserve-symlinks`
<!-- YAML
added: v6.3.0
-->

Instructs the module loader to preserve symbolic links when resolving and
caching modules.

By default, when Node.js loads a module from a path that is symbolically linked
to a different on-disk location, Node.js will dereference the link and use the
actual on-disk "real path" of the module as both an identifier and as a root
path to locate other dependency modules. In most cases, this default behavior
is acceptable. However, when using symbolically linked peer dependencies, as
illustrated in the example below, the default behavior causes an exception to
be thrown if `moduleA` attempts to require `moduleB` as a peer dependency:

```text
{appDir}
 ├── app
 │   ├── index.js
 │   └── node_modules
 │       ├── moduleA -> {appDir}/moduleA
 │       └── moduleB
 │           ├── index.js
 │           └── package.json
 └── moduleA
     ├── index.js
     └── package.json
```

The `--preserve-symlinks` command line flag instructs Node.js to use the
symlink path for modules as opposed to the real path, allowing symbolically
linked peer dependencies to be found.

Note, however, that using `--preserve-symlinks` can have other side effects.
Specifically, symbolically linked *native* modules can fail to load if those
are linked from more than one location in the dependency tree (Node.js would
see those as two separate modules and would attempt to load the module multiple
times, causing an exception to be thrown).

The `--preserve-symlinks` flag does not apply to the main module, which allows
`node --preserve-symlinks node_module/.bin/<foo>` to work. To apply the same
behavior for the main module, also use `--preserve-symlinks-main`.

### `--preserve-symlinks-main`
<!-- YAML
added: v10.2.0
-->

Instructs the module loader to preserve symbolic links when resolving and
caching the main module (`require.main`).

This flag exists so that the main module can be opted-in to the same behavior
that `--preserve-symlinks` gives to all other imports; they are separate flags,
however, for backward compatibility with older Node.js versions.

Note that `--preserve-symlinks-main` does not imply `--preserve-symlinks`; it
is expected that `--preserve-symlinks-main` will be used in addition to
`--preserve-symlinks` when it is not desirable to follow symlinks before
resolving relative paths.

See `--preserve-symlinks` for more information.

### `--prof`
<!-- YAML
added: v2.0.0
-->

Generate V8 profiler output.

### `--prof-process`
<!-- YAML
added: v5.2.0
-->

Process V8 profiler output generated using the V8 option `--prof`.

### `--redirect-warnings=file`
<!-- YAML
added: v8.0.0
-->

Write process warnings to the given file instead of printing to stderr. The
file will be created if it does not exist, and will be appended to if it does.
If an error occurs while attempting to write the warning to the file, the
warning will be written to stderr instead.

### `--throw-deprecation`
<!-- YAML
added: v0.11.14
-->

Throw errors for deprecations.

### `--title=title`
<!-- YAML
added: v10.7.0
-->

Set `process.title` on startup.

### `--tls-cipher-list=list`
<!-- YAML
added: v4.0.0
-->

Specify an alternative default TLS cipher list. Requires Node.js to be built
with crypto support (default).

### `--tls-max-v1.2`
<!-- YAML
added: REPLACEME
-->

Set [`tls.DEFAULT_MAX_VERSION`][] to 'TLSv1.2'. Use to disable support for
TLSv1.3.

### `--tls-max-v1.3`
<!-- YAML
added: REPLACEME
-->

Set default [`tls.DEFAULT_MAX_VERSION`][] to 'TLSv1.3'. Use to enable support
for TLSv1.3.

### `--tls-min-v1.0`
<!-- YAML
added: REPLACEME
-->

Set default [`tls.DEFAULT_MIN_VERSION`][] to 'TLSv1'. Use for compatibility with
old TLS clients or servers.

### `--tls-min-v1.1`
<!-- YAML
added: REPLACEME
-->

Set default [`tls.DEFAULT_MIN_VERSION`][] to 'TLSv1.1'. Use for compatibility
with old TLS clients or servers.

### `--tls-min-v1.3`
<!-- YAML
added: REPLACEME
-->

Set default [`tls.DEFAULT_MIN_VERSION`][] to 'TLSv1.3'. Use to disable support
for TLSv1.2, which is not as secure as TLSv1.3.

### `--trace-deprecation`
<!-- YAML
added: v0.8.0
-->

Print stack traces for deprecations.

### `--trace-event-categories`
<!-- YAML
added: v7.7.0
-->

A comma separated list of categories that should be traced when trace event
tracing is enabled using `--trace-events-enabled`.

### `--trace-event-file-pattern`
<!-- YAML
added: v9.8.0
-->

Template string specifying the filepath for the trace event data, it
supports `${rotation}` and `${pid}`.

### `--trace-events-enabled`
<!-- YAML
added: v7.7.0
-->

Enables the collection of trace event tracing information.

### `--trace-sync-io`
<!-- YAML
added: v2.1.0
-->

Prints a stack trace whenever synchronous I/O is detected after the first turn
of the event loop.

### `--trace-warnings`
<!-- YAML
added: v6.0.0
-->

Print stack traces for process warnings (including deprecations).

### `--track-heap-objects`
<!-- YAML
added: v2.4.0
-->

Track heap object allocations for heap snapshots.

### `--unhandled-rejections=mode`
<!-- YAML
added: REPLACEME
-->

By default all unhandled rejections trigger a warning plus a deprecation warning
for the very first unhandled rejection in case no [`unhandledRejection`][] hook
is used.

Using this flag allows to change what should happen when an unhandled rejection
occurs. One of three modes can be chosen:

* `strict`: Raise the unhandled rejection as an uncaught exception.
* `warn`: Always trigger a warning, no matter if the [`unhandledRejection`][]
  hook is set or not but do not print the deprecation warning.
* `none`: Silence all warnings.

### `--use-bundled-ca`, `--use-openssl-ca`
<!-- YAML
added: v6.11.0
-->

Use bundled Mozilla CA store as supplied by current Node.js version
or use OpenSSL's default CA store. The default store is selectable
at build-time.

The bundled CA store, as supplied by Node.js, is a snapshot of Mozilla CA store
that is fixed at release time. It is identical on all supported platforms.

Using OpenSSL store allows for external modifications of the store. For most
Linux and BSD distributions, this store is maintained by the distribution
maintainers and system administrators. OpenSSL CA store location is dependent on
configuration of the OpenSSL library but this can be altered at runtime using
environment variables.

See `SSL_CERT_DIR` and `SSL_CERT_FILE`.

### `--v8-options`
<!-- YAML
added: v0.1.3
-->

Print V8 command line options.

### `--v8-pool-size=num`
<!-- YAML
added: v5.10.0
-->

Set V8's thread pool size which will be used to allocate background jobs.

If set to `0` then V8 will choose an appropriate size of the thread pool based
on the number of online processors.

If the value provided is larger than V8's maximum, then the largest value
will be chosen.

### `--zero-fill-buffers`
<!-- YAML
added: v6.0.0
-->

Automatically zero-fills all newly allocated [`Buffer`][] and [`SlowBuffer`][]
instances.

### `-c`, `--check`
<!-- YAML
added:
  - v5.0.0
  - v4.2.0
changes:
  - version: v10.0.0
    pr-url: https://github.com/nodejs/node/pull/19600
    description: The `--require` option is now supported when checking a file.
-->

Syntax check the script without executing.

### `-e`, `--eval "script"`
<!-- YAML
added: v0.5.2
changes:
  - version: v5.11.0
    pr-url: https://github.com/nodejs/node/pull/5348
    description: Built-in libraries are now available as predefined variables.
-->

Evaluate the following argument as JavaScript. The modules which are
predefined in the REPL can also be used in `script`.

On Windows, using `cmd.exe` a single quote will not work correctly because it
only recognizes double `"` for quoting. In Powershell or Git bash, both `'`
and `"` are usable.

### `-h`, `--help`
<!-- YAML
added: v0.1.3
-->

Print node command line options.
The output of this option is less detailed than this document.

### `-i`, `--interactive`
<!-- YAML
added: v0.7.7
-->

Opens the REPL even if stdin does not appear to be a terminal.

### `-p`, `--print "script"`
<!-- YAML
added: v0.6.4
changes:
  - version: v5.11.0
    pr-url: https://github.com/nodejs/node/pull/5348
    description: Built-in libraries are now available as predefined variables.
-->

Identical to `-e` but prints the result.

### `-r`, `--require module`
<!-- YAML
added: v1.6.0
-->

Preload the specified module at startup.

Follows `require()`'s module resolution
rules. `module` may be either a path to a file, or a node module name.

### `-v`, `--version`
<!-- YAML
added: v0.1.3
-->

Print node's version.

## Environment Variables

### `NODE_DEBUG=module[,…]`
<!-- YAML
added: v0.1.32
-->

`','`-separated list of core modules that should print debug information.

### `NODE_DEBUG_NATIVE=module[,…]`

`','`-separated list of core C++ modules that should print debug information.

### `NODE_DISABLE_COLORS=1`
<!-- YAML
added: v0.3.0
-->

When set, colors will not be used in the REPL.

### `NODE_EXTRA_CA_CERTS=file`
<!-- YAML
added: v7.3.0
-->

When set, the well known "root" CAs (like VeriSign) will be extended with the
extra certificates in `file`. The file should consist of one or more trusted
certificates in PEM format. A message will be emitted (once) with
[`process.emitWarning()`][emit_warning] if the file is missing or
malformed, but any errors are otherwise ignored.

Note that neither the well known nor extra certificates are used when the `ca`
options property is explicitly specified for a TLS or HTTPS client or server.

This environment variable is ignored when `node` runs as setuid root or
has Linux file capabilities set.

### `NODE_ICU_DATA=file`
<!-- YAML
added: v0.11.15
-->

Data path for ICU (`Intl` object) data. Will extend linked-in data when compiled
with small-icu support.

### `NODE_NO_WARNINGS=1`
<!-- YAML
added: v6.11.0
-->

When set to `1`, process warnings are silenced.

### `NODE_OPTIONS=options...`
<!-- YAML
added: v8.0.0
-->

A space-separated list of command line options. `options...` are interpreted as
if they had been specified on the command line before the actual command line
(so they can be overridden). Node.js will exit with an error if an option
that is not allowed in the environment is used, such as `-p` or a script file.

Node.js options that are allowed are:
- `--diagnostic-report-directory`
- `--diagnostic-report-filename`
- `--diagnostic-report-on-fatalerror`
- `--diagnostic-report-on-signal`
- `--diagnostic-report-signal`
- `--diagnostic-report-uncaught-exception`
- `--enable-fips`
- `--experimental-modules`
- `--experimental-repl-await`
- `--experimental-report`
- `--experimental-vm-modules`
- `--force-fips`
- `--frozen-intrinsics`
- `--heapsnapshot-signal`
- `--icu-data-dir`
- `--inspect`
- `--inspect-brk`
- `--inspect-port`
- `--loader`
- `--max-http-header-size`
- `--napi-modules`
- `--no-deprecation`
- `--no-force-async-hooks-checks`
- `--no-warnings`
- `--openssl-config`
- `--pending-deprecation`
- `--redirect-warnings`
- `--require`, `-r`
- `--throw-deprecation`
- `--title`
- `--tls-cipher-list`
- `--trace-deprecation`
- `--trace-event-categories`
- `--trace-event-file-pattern`
- `--trace-events-enabled`
- `--trace-sync-io`
- `--trace-warnings`
- `--track-heap-objects`
- `--unhandled-rejections`
- `--use-bundled-ca`
- `--use-openssl-ca`
- `--v8-pool-size`
- `--zero-fill-buffers`

V8 options that are allowed are:
- `--abort-on-uncaught-exception`
- `--max-old-space-size`
- `--perf-basic-prof`
- `--perf-basic-prof-only-functions`
- `--perf-prof`
- `--perf-prof-unwinding-info`
- `--stack-trace-limit`

### `NODE_PATH=path[:…]`
<!-- YAML
added: v0.1.32
-->

`':'`-separated list of directories prefixed to the module search path.

On Windows, this is a `';'`-separated list instead.

### `NODE_PENDING_DEPRECATION=1`
<!-- YAML
added: v8.0.0
-->

When set to `1`, emit pending deprecation warnings.

Pending deprecations are generally identical to a runtime deprecation with the
notable exception that they are turned *off* by default and will not be emitted
unless either the `--pending-deprecation` command line flag, or the
`NODE_PENDING_DEPRECATION=1` environment variable, is set. Pending deprecations
are used to provide a kind of selective "early warning" mechanism that
developers may leverage to detect deprecated API usage.

### `NODE_PENDING_PIPE_INSTANCES=instances`

Set the number of pending pipe instance handles when the pipe server is waiting
for connections. This setting applies to Windows only.

### `NODE_PRESERVE_SYMLINKS=1`
<!-- YAML
added: v7.1.0
-->

When set to `1`, instructs the module loader to preserve symbolic links when
resolving and caching modules.

### `NODE_REDIRECT_WARNINGS=file`
<!-- YAML
added: v8.0.0
-->

When set, process warnings will be emitted to the given file instead of
printing to stderr. The file will be created if it does not exist, and will be
appended to if it does. If an error occurs while attempting to write the
warning to the file, the warning will be written to stderr instead. This is
equivalent to using the `--redirect-warnings=file` command-line flag.

### `NODE_REPL_HISTORY=file`
<!-- YAML
added: v3.0.0
-->

Path to the file used to store the persistent REPL history. The default path is
`~/.node_repl_history`, which is overridden by this variable. Setting the value
to an empty string (`''` or `' '`) disables persistent REPL history.

### `NODE_TLS_REJECT_UNAUTHORIZED=value`

If `value` equals `'0'`, certificate validation is disabled for TLS connections.
This makes TLS, and HTTPS by extension, insecure. The use of this environment
variable is strongly discouraged.

### `NODE_V8_COVERAGE=dir`

When set, Node.js will begin outputting [V8 JavaScript code coverage][] to the
directory provided as an argument. Coverage is output as an array of
[ScriptCoverage][] objects:

```json
{
  "result": [
    {
      "scriptId": "67",
      "url": "internal/tty.js",
      "functions": []
    }
  ]
}
```

`NODE_V8_COVERAGE` will automatically propagate to subprocesses, making it
easier to instrument applications that call the `child_process.spawn()` family
of functions. `NODE_V8_COVERAGE` can be set to an empty string, to prevent
propagation.

At this time coverage is only collected in the main thread and will not be
output for code executed by worker threads.

### `OPENSSL_CONF=file`
<!-- YAML
added: v6.11.0
-->

Load an OpenSSL configuration file on startup. Among other uses, this can be
used to enable FIPS-compliant crypto if Node.js is built with `./configure
--openssl-fips`.

If the [`--openssl-config`][] command line option is used, the environment
variable is ignored.

### `SSL_CERT_DIR=dir`
<!-- YAML
added: v7.7.0
-->

If `--use-openssl-ca` is enabled, this overrides and sets OpenSSL's directory
containing trusted certificates.

Be aware that unless the child environment is explicitly set, this environment
variable will be inherited by any child processes, and if they use OpenSSL, it
may cause them to trust the same CAs as node.

### `SSL_CERT_FILE=file`
<!-- YAML
added: v7.7.0
-->

If `--use-openssl-ca` is enabled, this overrides and sets OpenSSL's file
containing trusted certificates.

Be aware that unless the child environment is explicitly set, this environment
variable will be inherited by any child processes, and if they use OpenSSL, it
may cause them to trust the same CAs as node.

### `UV_THREADPOOL_SIZE=size`

Set the number of threads used in libuv's threadpool to `size` threads.

Asynchronous system APIs are used by Node.js whenever possible, but where they
do not exist, libuv's threadpool is used to create asynchronous node APIs based
on synchronous system APIs. Node.js APIs that use the threadpool are:

- all `fs` APIs, other than the file watcher APIs and those that are explicitly
  synchronous
- `crypto.pbkdf2()`
- `crypto.randomBytes()`, unless it is used without a callback
- `crypto.randomFill()`
- `dns.lookup()`
- all `zlib` APIs, other than those that are explicitly synchronous

Because libuv's threadpool has a fixed size, it means that if for whatever
reason any of these APIs takes a long time, other (seemingly unrelated) APIs
that run in libuv's threadpool will experience degraded performance. In order to
mitigate this issue, one potential solution is to increase the size of libuv's
threadpool by setting the `'UV_THREADPOOL_SIZE'` environment variable to a value
greater than `4` (its current default value). For more information, see the
[libuv threadpool documentation][].

[`--openssl-config`]: #cli_openssl_config_file
[`Buffer`]: buffer.html#buffer_class_buffer
[`SlowBuffer`]: buffer.html#buffer_class_slowbuffer
[`process.setUncaughtExceptionCaptureCallback()`]: process.html#process_process_setuncaughtexceptioncapturecallback_fn
[`tls.DEFAULT_MAX_VERSION`]: tls.html#tls_tls_default_max_version
[`tls.DEFAULT_MIN_VERSION`]: tls.html#tls_tls_default_min_version
[`unhandledRejection`]: process.html#process_event_unhandledrejection
[Chrome DevTools Protocol]: https://chromedevtools.github.io/devtools-protocol/
[REPL]: repl.html
[ScriptCoverage]: https://chromedevtools.github.io/devtools-protocol/tot/Profiler#type-ScriptCoverage
[V8 JavaScript code coverage]: https://v8project.blogspot.com/2017/12/javascript-code-coverage.html
[customizing esm specifier resolution]: esm.html#esm_customizing_esm_specifier_resolution_algorithm
[debugger]: debugger.html
[debugging security implications]: https://nodejs.org/en/docs/guides/debugging-getting-started/#security-implications
[emit_warning]: process.html#process_process_emitwarning_warning_type_code_ctor
[experimental ECMAScript Module]: esm.html#esm_resolve_hook
[libuv threadpool documentation]: http://docs.libuv.org/en/latest/threadpool.html
[remote code execution]: https://www.owasp.org/index.php/Code_Injection