Command-line options

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, see the debugger documentation.

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.

If an option that takes a single value (such as --max-http-header-size) is passed more than once, then the last passed value is used. Options from the command line take precedence over options passed through the NODE_OPTIONS environment variable.

-

Alias for stdin. Analogous to the use of - in other command-line utilities, meaning that the script is 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 is used as a script filename.

--abort-on-uncaught-exception

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

Print source-able bash completion script for Node.js.

$ node --completion-bash > node_bash_completion
$ source node_bash_completion

--conditions=condition

Enable experimental support for custom conditional exports resolution conditions.

Any number of custom string condition names are permitted.

The default Node.js conditions of "node", "default", "import", and "require" will always apply as defined.

--cpu-prof

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 is placed in the current working directory.

If --cpu-prof-name is not specified, the generated profile is named CPU.${yyyymmdd}.${hhmmss}.${pid}.${tid}.${seq}.cpuprofile.

$ node --cpu-prof index.js
$ ls *.cpuprofile
CPU.20190409.202950.15293.0.0.cpuprofile

--cpu-prof-dir

Specify the directory where the CPU profiles generated by --cpu-prof will be placed.

The default value is controlled by the --diagnostic-dir command-line option.

--cpu-prof-interval

Specify the sampling interval in microseconds for the CPU profiles generated by --cpu-prof. The default is 1000 microseconds.

--cpu-prof-name

Specify the file name of the CPU profile generated by --cpu-prof.

--diagnostic-dir=directory

Set the directory to which all diagnostic output files are written. Defaults to current working directory.

Affects the default output directory of:

• --cpu-prof-dir
• --heap-prof-dir
• --redirect-warnings

--disable-proto=mode

Disable the Object.prototype.__proto__ property. If mode is delete, the property is removed entirely. If mode is throw, accesses to the property throw an exception with the code ERR_PROTO_ACCESS.

--disallow-code-generation-from-strings

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

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

--enable-source-maps

Enable experimental Source Map v3 support for stack traces.

Currently, overriding Error.prepareStackTrace is ignored when the --enable-source-maps flag is set.

--experimental-import-meta-resolve

Enable experimental import.meta.resolve() support.

--experimental-json-modules

Enable experimental JSON support for the ES Module loader.

--experimental-loader=module

Specify the module of a custom experimental ECMAScript Module loader. module may be either a path to a file, or an ECMAScript Module name.

--experimental-modules

Enable latest experimental modules features (deprecated).

--experimental-policy

Use the specified file as a security policy.

--experimental-repl-await

Enable experimental top-level await keyword support in REPL.

--experimental-specifier-resolution=mode

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 enables support for optional file extensions and the ability to import a directory that has an index file.

See customizing ESM specifier resolution for example usage.

--experimental-vm-modules

Enable experimental ES Module support in the vm module.

--experimental-wasi-unstable-preview1

Enable experimental WebAssembly System Interface (WASI) support.

--experimental-wasm-modules

--force-context-aware

Disable loading native addons that are not context-aware.

Enable experimental WebAssembly module support.

--force-fips

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

--frozen-intrinsics

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.

--heapsnapshot-signal=signal

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.heapsnapshot

--heap-prof

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 is placed in the current working directory.

If --heap-prof-name is not specified, the generated profile is named Heap.${yyyymmdd}.${hhmmss}.${pid}.${tid}.${seq}.heapprofile.

$ node --heap-prof index.js
$ ls *.heapprofile
Heap.20190409.202950.15293.0.001.heapprofile

--heap-prof-dir

Specify the directory where the heap profiles generated by --heap-prof will be placed.

The default value is controlled by the --diagnostic-dir command-line option.

--heap-prof-interval

Specify the average sampling interval in bytes for the heap profiles generated by --heap-prof. The default is 512 * 1024 bytes.

--heap-prof-name

Specify the file name of the heap profile generated by --heap-prof.

--icu-data-dir=file

Specify ICU data load path. (Overrides NODE_ICU_DATA.)

--input-type=type

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".

--inspect-brk[=[host:]port]

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

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.

--inspect[=[host:]port]

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.

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.

--inspect-publish-uid=stderr,http

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.

--insecure-http-parser

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.

--jitless

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.

--max-http-header-size=size

Specify the maximum size, in bytes, of HTTP headers. Defaults to 16KB.

--napi-modules

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

--no-deprecation

Silence deprecation warnings.

--no-force-async-hooks-checks

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

--no-warnings

Silence all process warnings (including deprecations).

--openssl-config=file

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

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.

--policy-integrity=sri

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.

--preserve-symlinks

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.

--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; use --preserve-symlinks-main in addition to --preserve-symlinks when it is not desirable to follow symlinks before resolving relative paths.

See --preserve-symlinks for more information.

--prof

Generate V8 profiler output.

--prof-process

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

--redirect-warnings=file

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.

The file name may be an absolute path. If it is not, the default directory it will be written to is controlled by the --diagnostic-dir command-line option.

--report-compact

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.

--report-dir=directory, report-directory=directory

Location at which the report will be generated.

--report-filename=filename

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

--report-on-fatalerror

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.

--report-on-signal

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.

--report-signal=signal

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

--report-uncaught-exception

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-deprecation

Throw errors for deprecations.

--title=title

Set process.title on startup.

--tls-cipher-list=list

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

--tls-keylog=file

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.

--tls-max-v1.2

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

--tls-max-v1.3

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

--tls-min-v1.0

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

--tls-min-v1.1

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

--tls-min-v1.2

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.

--tls-min-v1.3

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-atomics-wait

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 SharedArrayBuffer in question, as well as the byte offset corresponding to the index passed to Atomics.wait()
• The expected value that was passed to Atomics.wait()
• The timeout passed to Atomics.wait

--trace-deprecation

Print stack traces for deprecations.

--trace-event-categories

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

--trace-event-file-pattern

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

--trace-events-enabled

Enables the collection of trace event tracing information.

--trace-exit

Prints a stack trace whenever an environment is exited proactively, i.e. invoking process.exit().

--trace-sigint

Prints a stack trace on SIGINT.

--trace-sync-io

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

--trace-tls

Prints TLS packet trace information to stderr. This can be used to debug TLS connection problems.

--trace-uncaught

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.

--trace-warnings

Print stack traces for process warnings (including deprecations).

--track-heap-objects

Track heap object allocations for heap snapshots.

--unhandled-rejections=mode

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 the following modes can be chosen:

• throw: Emit unhandledRejection. If this hook is not set, raise the unhandled rejection as an uncaught exception.
• 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.
• warn-with-error-code: Emit unhandledRejection. If this hook is not set, trigger a warning, and set the process exit code to 1.
• none: Silence all warnings.

--use-bundled-ca, --use-openssl-ca

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.

--use-largepages=mode

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.

--v8-options

Print V8 command-line options.

--v8-pool-size=num

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

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

-c, --check

Syntax check the script without executing.

-e, --eval "script"

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

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

-i, --interactive

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

-p, --print "script"

Identical to -e but prints the result.

-r, --require module

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

Print node's version.

Environment variables

NODE_DEBUG=module[,…]

','-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

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

NODE_EXTRA_CA_CERTS=file

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.

NODE_ICU_DATA=file

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

NODE_NO_WARNINGS=1

When set to 1, process warnings are silenced.

NODE_OPTIONS=options...

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.

If an option value contains a space, it can be escaped using double quotes:

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:5555

A 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:

• --conditions
• --diagnostic-dir
• --disable-proto
• --enable-fips
• --enable-source-maps
• --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.

NODE_PATH=path[:…]

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

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

NODE_PENDING_DEPRECATION=1

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

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

NODE_REDIRECT_WARNINGS=file

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

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_REPL_EXTERNAL_MODULE=file

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.

NODE_SKIP_PLATFORM_CHECK=value

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.

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 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 output

Coverage is output as an array of ScriptCoverage objects on the top-level key result:

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

Source map cache

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
      ]
    }
  }
}

OPENSSL_CONF=file

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

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

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
• asynchronous crypto APIs such as crypto.pbkdf2(), crypto.scrypt(), crypto.randomBytes(), crypto.randomFill(), crypto.generateKeyPair()
• 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.

Useful V8 options

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:

--max-old-space-size=SIZE (in megabytes)

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