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.
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.
All options, including V8 options, allow words to be separated by both
dashes (-) or underscores (_).
For example, --pending-deprecation is equivalent to --pending_deprecation.
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.
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.
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).
Print source-able bash completion script for Node.js.
$ node --completion-bash > node_bash_completion
$ source node_bash_completionStability: 1 - Experimental
Starts the V8 CPU profiler on start up, and writes the CPU profile to disk before exit.
If --cpu-prof-dir is not specified, the generated profile will be placed
in the current working directory.
If --cpu-prof-name is not specified, the generated profile will be
named CPU.${yyyymmdd}.${hhmmss}.${pid}.${tid}.${seq}.cpuprofile.
$ node --cpu-prof index.js
$ ls *.cpuprofile
CPU.20190409.202950.15293.0.0.cpuprofileStability: 1 - Experimental
Specify the directory where the CPU profiles generated by --cpu-prof will
be placed.
Stability: 1 - Experimental
Specify the sampling interval in microseconds for the CPU profiles generated
by --cpu-prof. The default is 1000 microseconds.
Stability: 1 - Experimental
Specify the file name of the CPU profile generated by --cpu-prof.
Disable the Object.prototype.__proto__ property. If mode is delete, the
property will be removed entirely. If mode is throw, accesses to the
property will throw an exception with the code ERR_PROTO_ACCESS.
Make built-in language features like eval and new Function that generate
code from strings throw an exception instead. This does not affect the Node.js
vm module.
Enable FIPS-compliant crypto at startup. (Requires Node.js to be built with
./configure --openssl-fips.)
Stability: 1 - Experimental
Enable experimental Source Map V3 support for stack traces.
Currently, overriding Error.prepareStackTrace is ignored when the
--enable-source-maps flag is set.
Experimental AbortController and AbortSignal support is enabled by default.
Use of this command line flag is no longer required.
Enable experimental import.meta.resolve() support.
Enable experimental JSON support for the ES Module loader.
Specify the module of a custom experimental ECMAScript Module loader.
module may be either a path to a file, or an ECMAScript Module name.
Enable latest experimental modules features (deprecated).
Use the specified file as a security policy.
Enable experimental top-level await keyword support in REPL.
Sets the resolution algorithm for resolving ES module 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.
Enable experimental top-level await keyword support, available only in ES
module scripts.
(See also --experimental-repl-await.)
Enable experimental ES Module support in the vm module.
Enable experimental WebAssembly System Interface (WASI) support.
Disable loading native addons that are not context-aware.
Enable experimental WebAssembly module support.
Force FIPS-compliant crypto on startup. (Cannot be disabled from script code.)
(Same requirements as --enable-fips.)
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 may break under this flag.
--require runs prior to freezing intrinsics in order to allow polyfills to
be added.
Enables a signal handler that causes the Node.js process to write a heap dump
when the specified signal is received. signal must be a valid signal name.
Disabled by default.
$ node --heapsnapshot-signal=SIGUSR2 index.js &
$ ps aux
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
node 1 5.5 6.1 787252 247004 ? Ssl 16:43 0:02 node --heapsnapshot-signal=SIGUSR2 index.js
$ kill -USR2 1
$ ls
Heap.20190718.133405.15554.0.001.heapsnapshotStability: 1 - Experimental
Starts the V8 heap profiler on start up, and writes the heap profile to disk before exit.
If --heap-prof-dir is not specified, the generated profile will be placed
in the current working directory.
If --heap-prof-name is not specified, the generated profile will be
named Heap.${yyyymmdd}.${hhmmss}.${pid}.${tid}.${seq}.heapprofile.
$ node --heap-prof index.js
$ ls *.heapprofile
Heap.20190409.202950.15293.0.001.heapprofileStability: 1 - Experimental
Specify the directory where the heap profiles generated by --heap-prof will
be placed.
Stability: 1 - Experimental
Specify the average sampling interval in bytes for the heap profiles generated
by --heap-prof. The default is 512 * 1024 bytes.
Stability: 1 - Experimental
Specify the file name of the heap profile generated by --heap-prof.
Specify ICU data load path. (Overrides NODE_ICU_DATA.)
This configures Node.js to interpret string input as CommonJS or as an ES
module. String input is input via --eval, --print, or STDIN.
Valid values are "commonjs" and "module". The default is "commonjs".
Activate inspector on host:port and break at start of user script.
Default host:port is 127.0.0.1:9229.
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 below regarding the host
parameter usage.
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.
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.
Specify ways of the inspector web socket url exposure.
By default inspector websocket url is available in stderr and under /json/list
endpoint on http://host:port/json/list.
Use an insecure HTTP parser that accepts invalid HTTP headers. This may allow interoperability with non-conformant HTTP implementations. It may also allow request smuggling and other HTTP attacks that rely on invalid headers being accepted. Avoid using this option.
Disable runtime allocation of executable memory. This may be required on some platforms for security reasons. It can also reduce attack surface on other platforms, but the performance impact may be severe.
This flag is inherited from V8 and is subject to change upstream. It may disappear in a non-semver-major release.
Specify the maximum size, in bytes, of HTTP headers. Defaults to 16KB.
This option is a no-op. It is kept for compatibility.
Silence deprecation warnings.
Disables runtime checks for async_hooks. These will still be enabled
dynamically when async_hooks is enabled.
Silence all process warnings (including deprecations).
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.
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.
Stability: 1 - Experimental
Instructs Node.js to error prior to running any code if the policy does not have the specified integrity. It expects a Subresource Integrity string as a parameter.
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:
{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.
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.
--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.
Generate V8 profiler output.
Process V8 profiler output generated using the V8 option --prof.
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.
Write reports in a compact format, single-line JSON, more easily consumable by log processing systems than the default multi-line format designed for human consumption.
Location at which the report will be generated.
Name of the file to which the report will be written.
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. Useful to inspect various diagnostic data elements such as heap, stack, event loop state, resource consumption etc. to reason about the fatal error.
Enables report to be generated upon receiving the specified (or predefined)
signal to the running Node.js process. The signal to trigger the report is
specified through --report-signal.
Sets or resets the signal for report generation (not supported on Windows).
Default signal is SIGUSR2.
Enables report to be generated on uncaught exceptions. Useful when inspecting the JavaScript stack in conjunction with native stack and other runtime environment data.
Throw errors for deprecations.
Set process.title on startup.
Specify an alternative default TLS cipher list. Requires Node.js to be built with crypto support (default).
Log TLS key material to a file. The key material is in NSS SSLKEYLOGFILE
format and can be used by software (such as Wireshark) to decrypt the TLS
traffic.
Set tls.DEFAULT_MAX_VERSION to 'TLSv1.2'. Use to disable support for
TLSv1.3.
Set default tls.DEFAULT_MAX_VERSION to 'TLSv1.3'. Use to enable support
for TLSv1.3.
Set default tls.DEFAULT_MIN_VERSION to 'TLSv1'. Use for compatibility with
old TLS clients or servers.
Set default tls.DEFAULT_MIN_VERSION to 'TLSv1.1'. Use for compatibility
with old TLS clients or servers.
Set default tls.DEFAULT_MIN_VERSION to 'TLSv1.2'. This is the default for
12.x and later, but the option is supported for compatibility with older Node.js
versions.
Set default tls.DEFAULT_MIN_VERSION to 'TLSv1.3'. Use to disable support
for TLSv1.2, which is not as secure as TLSv1.3.
Print short summaries of calls to Atomics.wait() to stderr.
The output could look like this:
(node:15701) [Thread 0] Atomics.wait(<address> + 0, 1, inf) started
(node:15701) [Thread 0] Atomics.wait(<address> + 0, 1, inf) did not wait because the values mismatched
(node:15701) [Thread 0] Atomics.wait(<address> + 0, 0, 10) started
(node:15701) [Thread 0] Atomics.wait(<address> + 0, 0, 10) timed out
(node:15701) [Thread 0] Atomics.wait(<address> + 4, 0, inf) started
(node:15701) [Thread 1] Atomics.wait(<address> + 4, -1, inf) started
(node:15701) [Thread 0] Atomics.wait(<address> + 4, 0, inf) was woken up by another thread
(node:15701) [Thread 1] Atomics.wait(<address> + 4, -1, inf) was woken up by another thread
The fields here correspond to:
- The thread id as given by
worker_threads.threadId - The base address of the
SharedArrayBufferin question, as well as the byte offset corresponding to the index passed toAtomics.wait() - The expected value that was passed to
Atomics.wait() - The timeout passed to
Atomics.wait
Print stack traces for deprecations.
A comma separated list of categories that should be traced when trace event
tracing is enabled using --trace-events-enabled.
Template string specifying the filepath for the trace event data, it
supports ${rotation} and ${pid}.
Enables the collection of trace event tracing information.
Prints a stack trace whenever an environment is exited proactively,
i.e. invoking process.exit().
Prints a stack trace on SIGINT.
Prints a stack trace whenever synchronous I/O is detected after the first turn of the event loop.
Prints TLS packet trace information to stderr. This can be used to debug TLS
connection problems.
Print stack traces for uncaught exceptions; usually, the stack trace associated
with the creation of an Error is printed, whereas this makes Node.js also
print the stack trace associated with throwing the value (which does not need
to be an Error instance).
Enabling this option may affect garbage collection behavior negatively.
Print stack traces for process warnings (including deprecations).
Track heap object allocations for heap snapshots.
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 theunhandledRejectionhook is set or not but do not print the deprecation warning.none: Silence all warnings.
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.
Re-map the Node.js static code to large memory pages at startup. If supported on the target system, this will cause the Node.js static code to be moved onto 2 MiB pages instead of 4 KiB pages.
The following values are valid for mode:
off: No mapping will be attempted. This is the default.on: If supported by the OS, mapping will be attempted. Failure to map will be ignored and a message will be printed to standard error.silent: If supported by the OS, mapping will be attempted. Failure to map will be ignored and will not be reported.
Print V8 command line options.
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.
Automatically zero-fills all newly allocated Buffer and SlowBuffer
instances.
Syntax check the script without executing.
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.
Print node command line options. The output of this option is less detailed than this document.
Opens the REPL even if stdin does not appear to be a terminal.
Identical to -e but prints the result.
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.
Print node's version.
','-separated list of core modules that should print debug information.
','-separated list of core C++ modules that should print debug information.
When set, colors will not be used in the REPL.
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() if the file is missing or
malformed, but any errors are otherwise ignored.
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.
Data path for ICU (Intl object) data. Will extend linked-in data when compiled
with small-icu support.
When set to 1, process warnings are silenced.
A space-separated list of command line options. options... are interpreted
before command line options, so command line options will override or
compound after anything in options.... 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.
In case an option value happens to contain a space (for example a path listed
in --require), it must be escaped using double quotes. For example:
NODE_OPTIONS='--require "./my path/file.js"'A singleton flag passed as a command line option will override the same flag
passed into NODE_OPTIONS:
# The inspector will be available on port 5555
NODE_OPTIONS='--inspect=localhost:4444' node --inspect=localhost:5555A flag that can be passed multiple times will be treated as if its
NODE_OPTIONS instances were passed first, and then its command line
instances afterwards:
NODE_OPTIONS='--require "./a.js"' node --require "./b.js"
# is equivalent to:
node --require "./a.js" --require "./b.js"Node.js options that are allowed are:
--disable-proto--enable-fips--enable-source-maps--experimental-abortcontroller--experimental-import-meta-resolve--experimental-json-modules--experimental-loader--experimental-modules--experimental-policy--experimental-repl-await--experimental-specifier-resolution--experimental-top-level-await--experimental-vm-modules--experimental-wasi-unstable-preview1--experimental-wasm-modules--force-context-aware--force-fips--frozen-intrinsics--heapsnapshot-signal--http-parser--icu-data-dir--input-type--insecure-http-parser--inspect-brk--inspect-port,--debug-port--inspect-publish-uid--inspect--max-http-header-size--napi-modules--no-deprecation--no-force-async-hooks-checks--no-warnings--openssl-config--pending-deprecation--policy-integrity--preserve-symlinks-main--preserve-symlinks--prof-process--redirect-warnings--report-compact--report-dir,--report-directory--report-filename--report-on-fatalerror--report-on-signal--report-signal--report-uncaught-exception--require,-r--throw-deprecation--title--tls-cipher-list--tls-keylog--tls-max-v1.2--tls-max-v1.3--tls-min-v1.0--tls-min-v1.1--tls-min-v1.2--tls-min-v1.3--trace-atomics-wait--trace-deprecation--trace-event-categories--trace-event-file-pattern--trace-events-enabled--trace-exit--trace-sigint--trace-sync-io--trace-tls--trace-uncaught--trace-warnings--track-heap-objects--unhandled-rejections--use-bundled-ca--use-largepages--use-openssl-ca--v8-pool-size--zero-fill-buffers
V8 options that are allowed are:
--abort-on-uncaught-exception--disallow-code-generation-from-strings--huge-max-old-generation-size--interpreted-frames-native-stack--jitless--max-old-space-size--perf-basic-prof-only-functions--perf-basic-prof--perf-prof-unwinding-info--perf-prof--stack-trace-limit
--perf-basic-prof-only-functions, --perf-basic-prof,
--perf-prof-unwinding-info, and --perf-prof are only available on Linux.
':'-separated list of directories prefixed to the module search path.
On Windows, this is a ';'-separated list instead.
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.
Set the number of pending pipe instance handles when the pipe server is waiting for connections. This setting applies to Windows only.
When set to 1, instructs the module loader to preserve symbolic links when
resolving and caching modules.
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.
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.
Path to a Node.js module which will be loaded in place of the built-in REPL.
Overriding this value to an empty string ('') will use the built-in REPL.
If value equals '1', the check for a supported platform is skipped during
Node.js startup. Node.js might not execute correctly. Any issues encountered
on unsupported platforms will not be fixed.
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.
When set, Node.js will begin outputting V8 JavaScript code coverage and
Source Map data to the directory provided as an argument (coverage
information is written as JSON to files with a coverage prefix).
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.
Coverage is output as an array of ScriptCoverage objects on the top-level
key result:
{
"result": [
{
"scriptId": "67",
"url": "internal/tty.js",
"functions": []
}
]
}Stability: 1 - Experimental
If found, Source Map data is appended to the top-level key source-map-cache
on the JSON coverage object.
source-map-cache is an object with keys representing the files source maps
were extracted from, and values which include the raw source-map URL
(in the key url), the parsed Source Map V3 information (in the key data),
and the line lengths of the source file (in the key lineLengths).
{
"result": [
{
"scriptId": "68",
"url": "file:///absolute/path/to/source.js",
"functions": []
}
],
"source-map-cache": {
"file:///absolute/path/to/source.js": {
"url": "./path-to-map.json",
"data": {
"version": 3,
"sources": [
"file:///absolute/path/to/original.js"
],
"names": [
"Foo",
"console",
"info"
],
"mappings": "MAAMA,IACJC,YAAaC",
"sourceRoot": "./"
},
"lineLengths": [
13,
62,
38,
27
]
}
}
}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.
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.
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.
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
fsAPIs, other than the file watcher APIs and those that are explicitly synchronous - asynchronous crypto APIs such as
crypto.pbkdf2(),crypto.scrypt(),crypto.randomBytes(),crypto.randomFill(),crypto.generateKeyPair() dns.lookup()- all
zlibAPIs, 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.
V8 has its own set of CLI options. Any V8 CLI option that is provided to node
will be passed on to V8 to handle. V8's options have no stability guarantee.
The V8 team themselves don't consider them to be part of their formal API,
and reserve the right to change them at any time. Likewise, they are not
covered by the Node.js stability guarantees. Many of the V8
options are of interest only to V8 developers. Despite this, there is a small
set of V8 options that are widely applicable to Node.js, and they are
documented here:
Sets the max memory size of V8's old memory section. As memory consumption approaches the limit, V8 will spend more time on garbage collection in an effort to free unused memory.
On a machine with 2GB of memory, consider setting this to 1536 (1.5GB) to leave some memory for other uses and avoid swapping.
$ node --max-old-space-size=1536 index.js