Node.js includes a command-line debugging utility. The Node.js debugger client\nis not a full-featured debugger, but simple stepping and inspection are\npossible.
\nThe debugger supports two modes of operation: interactive mode and non-interactive probe mode.
", "miscs": [ { "textRaw": "Interactive mode", "name": "interactive_mode", "type": "misc", "desc": "$ node inspect [--port=<port>] [<node-option> ...] [<script> [<script-args>] | <host>:<port> | -p <pid>]\nTo use it, start Node.js with the inspect argument followed by the path to the\nscript to debug.
$ node inspect myscript.js\n< Debugger listening on ws://127.0.0.1:9229/621111f9-ffcb-4e82-b718-48a145fa5db8\n< For help, see: https://nodejs.org/learn/getting-started/debugging\n<\nconnecting to 127.0.0.1:9229 ... ok\n< Debugger attached.\n<\n ok\nBreak on start in myscript.js:2\n 1 // myscript.js\n> 2 global.x = 5;\n 3 setTimeout(() => {\n 4 debugger;\ndebug>\nThe debugger automatically breaks on the first executable line. To instead\nrun until the first breakpoint (specified by a debugger statement), set\nthe NODE_INSPECT_RESUME_ON_START environment variable to 1.
$ cat myscript.js\n// myscript.js\nglobal.x = 5;\nsetTimeout(() => {\n debugger;\n console.log('world');\n}, 1000);\nconsole.log('hello');\n$ NODE_INSPECT_RESUME_ON_START=1 node inspect myscript.js\n< Debugger listening on ws://127.0.0.1:9229/f1ed133e-7876-495b-83ae-c32c6fc319c2\n< For help, see: https://nodejs.org/learn/getting-started/debugging\n<\nconnecting to 127.0.0.1:9229 ... ok\n< Debugger attached.\n<\n< hello\n<\nbreak in myscript.js:4\n 2 global.x = 5;\n 3 setTimeout(() => {\n> 4 debugger;\n 5 console.log('world');\n 6 }, 1000);\ndebug> next\nbreak in myscript.js:5\n 3 setTimeout(() => {\n 4 debugger;\n> 5 console.log('world');\n 6 }, 1000);\n 7 console.log('hello');\ndebug> repl\nPress Ctrl+C to leave debug repl\n> x\n5\n> 2 + 2\n4\ndebug> next\n< world\n<\nbreak in myscript.js:6\n 4 debugger;\n 5 console.log('world');\n> 6 }, 1000);\n 7 console.log('hello');\n 8\ndebug> .exit\n$\nThe repl command allows code to be evaluated remotely. The next command\nsteps to the next line. Type help to see what other commands are available.
Pressing enter without typing a command will repeat the previous debugger\ncommand.
It is possible to watch expression and variable values while debugging. On\nevery breakpoint, each expression from the watchers list will be evaluated\nin the current context and displayed immediately before the breakpoint's\nsource code listing.
\nTo begin watching an expression, type watch('my_expression'). The command\nwatchers will print the active watchers. To remove a watcher, type\nunwatch('my_expression').
cont, c: Continue executionnext, n: Step nextstep, s: Step inout, o: Step outpause: Pause running code (like pause button in Developer Tools)setBreakpoint(), sb(): Set breakpoint on current linesetBreakpoint(line), sb(line): Set breakpoint on specific linesetBreakpoint('fn()'), sb(...): Set breakpoint on a first statement in\nfunction's bodysetBreakpoint('script.js', 1), sb(...): Set breakpoint on first line of\nscript.jssetBreakpoint('script.js', 1, 'num < 4'), sb(...): Set conditional\nbreakpoint on first line of script.js that only breaks when num < 4\nevaluates to trueclearBreakpoint('script.js', 1), cb(...): Clear breakpoint in script.js\non line 1It is also possible to set a breakpoint in a file (module) that\nis not loaded yet:
\n$ node inspect main.js\n< Debugger listening on ws://127.0.0.1:9229/48a5b28a-550c-471b-b5e1-d13dd7165df9\n< For help, see: https://nodejs.org/learn/getting-started/debugging\n<\nconnecting to 127.0.0.1:9229 ... ok\n< Debugger attached.\n<\nBreak on start in main.js:1\n> 1 const mod = require('./mod.js');\n 2 mod.hello();\n 3 mod.hello();\ndebug> setBreakpoint('mod.js', 22)\nWarning: script 'mod.js' was not loaded yet.\ndebug> c\nbreak in mod.js:22\n 20 // USE OR OTHER DEALINGS IN THE SOFTWARE.\n 21\n>22 exports.hello = function() {\n 23 return 'hello from module';\n 24 };\ndebug>\nIt is also possible to set a conditional breakpoint that only breaks when a\ngiven expression evaluates to true:
$ node inspect main.js\n< Debugger listening on ws://127.0.0.1:9229/ce24daa8-3816-44d4-b8ab-8273c8a66d35\n< For help, see: https://nodejs.org/learn/getting-started/debugging\n<\nconnecting to 127.0.0.1:9229 ... ok\n< Debugger attached.\nBreak on start in main.js:7\n 5 }\n 6\n> 7 addOne(10);\n 8 addOne(-1);\n 9\ndebug> setBreakpoint('main.js', 4, 'num < 0')\n 1 'use strict';\n 2\n 3 function addOne(num) {\n> 4 return num + 1;\n 5 }\n 6\n 7 addOne(10);\n 8 addOne(-1);\n 9\ndebug> cont\nbreak in main.js:4\n 2\n 3 function addOne(num) {\n> 4 return num + 1;\n 5 }\n 6\ndebug> exec('num')\n-1\ndebug>\nbacktrace, bt: Print backtrace of current execution framelist(5): List scripts source code with 5 line context (5 lines before and\nafter)watch(expr): Add expression to watch listunwatch(expr): Remove expression from watch listunwatch(index): Remove expression at specific index from watch listwatchers: List all watchers and their values (automatically listed on each\nbreakpoint)repl: Open debugger's repl for evaluation in debugging script's contextexec expr, p expr: Execute an expression in debugging script's context and\nprint its valueprofile: Start CPU profiling sessionprofileEnd: Stop current CPU profiling sessionprofiles: List all completed CPU profiling sessionsprofiles[n].save(filepath = 'node.cpuprofile'): Save CPU profiling session\nto disk as JSONtakeHeapSnapshot(filepath = 'node.heapsnapshot'): Take a heap snapshot\nand save to disk as JSONrun: Run script (automatically runs on debugger's start)restart: Restart scriptkill: Kill scriptscripts: List all loaded scriptsversion: Display V8's versionnode inspect supports a non-interactive probe mode for inspecting runtime values\nin an application via the flag --probe.
Currently, probe mode only supports launching a new process from the entry point\nscript specified on the command line.
\nThe probe mode sets one or more source breakpoints, evaluates specified\nexpressions whenever the execution reaches a breakpoint, and prints one\nfinal report of all the evaluated expressions when the session ends\n(either on normal completion, error, or timeout). This allows developers to perform\nprintf-style debugging without having to modify the application code and\nclean up afterwards. It also supports structured JSON output for tool use.
\n$ node inspect --probe <file>:<line>[:<col>] --expr <expr>\n [--probe <file>:<line>[:<col>] --expr <expr> ...]\n [--json] [--preview] [--timeout=<ms>] [--port=<port>]\n [--] [<node-option> ...] <script> [<script-args> ...]\n--probe <file>:<line>[:<col>]: Source location of the probe. When execution\nreaches the location, the provided expressions are evaluated and printed in\nthe output. Line and column numbers are 1-based. When omitted, column defaults to 1.--expr <expr>: JavaScript expression to evaluate whenever execution reaches\nthe location specified by the preceding --probe.\nMust immediately follow the --probe it belongs to.--timeout=<ms>: A global wall-clock deadline for the entire probe session.\nThe default is 30000. This can be used to probe a long-running application\nthat can be terminated externally.--json: If used, prints a structured JSON report instead of the default text report.--preview: If used, non-primitive values will include CDP property previews for\nobject-like JSON probe values.--port=<port>: Selects the local inspector port where the probing session\nwill listen. Defaults to 0, which requests a random port.-- is optional unless the child needs its own Node.js flags.Additional rules about the --probe and --expr arguments:
--probe <file>:<line>[:<col>] and --expr <expr> are strict pairs. Each\n--probe must be followed immediately by exactly one --expr.--timeout, --json, --preview, and --port are global probe options\nfor the whole probe session. They may appear before or between probe pairs,\nbut not between a --probe and its matching --expr.-- must be used to separate the probe options from the Node.js\noptions for the child script.Example:
\n$ node inspect --probe app.js:10 --expr \"user\"\n --probe src/utils.js:5:15 --expr \"config.options\"\n --json --preview -- --no-warnings app.js --arg-for-app=foo\nWhen the probe session ends, the probing process prints a final report of all the probe hits and results.
\nConsider this script:
\n// cli.js\nlet maxRSS = 0;\nfor (let i = 0; i < 2; i++) {\n const { rss } = process.memoryUsage();\n maxRSS = Math.max(maxRSS, rss);\n}\nWithout --json, by default the output is printed in a human-readable text format:
$ node inspect --probe cli.js:5 --expr 'rss' cli.js\nHit 1 at cli.js:5\n rss = 54935552\nHit 2 at cli.js:5\n rss = 55083008\nCompleted\nPrimitive results are printed directly, while objects and arrays use Chrome\nDevTools Protocol preview data when available. Other non-primitive values\nfall back to the Chrome DevTools Protocol description string.\nExpression failures are recorded as [error] ... lines and do not fail\nthe overall session. If richer text formatting is needed, wrap the expression\nin JSON.stringify(...) or util.inspect(...).
When --json is used, the output shape looks like this:
$ node inspect --json --probe cli.js:5 --expr 'rss' cli.js\n{\"v\":1,\"probes\":[{\"expr\":\"rss\",\"target\":[\"cli.js\",5]}],\"results\":[{\"probe\":0,\"event\":\"hit\",\"hit\":1,\"result\":{\"type\":\"number\",\"value\":55443456,\"description\":\"55443456\"}},{\"probe\":0,\"event\":\"hit\",\"hit\":2,\"result\":{\"type\":\"number\",\"value\":55574528,\"description\":\"55574528\"}},{\"event\":\"completed\"}]}\n{\n \"v\": 1, // Probe JSON schema version.\n \"probes\": [\n {\n \"expr\": \"rss\", // The expression paired with --probe.\n \"target\": [\"cli.js\", 5] // [file, line] or [file, line, col].\n }\n ],\n \"results\": [\n {\n \"probe\": 0, // Index into probes[].\n \"event\": \"hit\", // Hit events are recorded in observation order.\n \"hit\": 1, // 1-based hit count for this probe.\n \"result\": {\n \"type\": \"number\",\n \"value\": 55443456,\n \"description\": \"55443456\"\n }\n // If the expression throws, \"error\" is present instead of \"result\".\n },\n {\n \"probe\": 0,\n \"event\": \"hit\",\n \"hit\": 2,\n \"result\": {\n \"type\": \"number\",\n \"value\": 55574528,\n \"description\": \"55574528\"\n }\n },\n {\n \"event\": \"completed\"\n // The final entry is always a terminal event, for example:\n // 1. { \"event\": \"completed\" }\n // 2. { \"event\": \"miss\", \"pending\": [0, 1] }\n // 3. {\n // \"event\": \"timeout\",\n // \"pending\": [0],\n // \"error\": {\n // \"code\": \"probe_timeout\",\n // \"message\": \"Timed out after 30000ms waiting for probes: app.js:10\"\n // }\n // }\n // 4. {\n // \"event\": \"error\",\n // \"pending\": [0],\n // \"error\": {\n // \"code\": \"probe_target_exit\",\n // \"exitCode\": 1,\n // \"stderr\": \"[Error: boom]\",\n // \"message\": \"Target exited with code 1 before probes: app.js:10\"\n // }\n // }\n }\n ]\n}\nProbe mode only prints the final probe report to stdout, and otherwise silences\nstdout/stderr from the child process. When the probing session ends,\nnode inspect typically exits with code 0 and prints a final report to\nstdout. If the child process exits with a non-zero code before the\nprobe session ends, the final report records a terminal error event along\nwith the exit code and captured child stderr. The probing process itself\nstill exits with code 0 in this case.
Invalid arguments and fatal launch or connect failures may cause the\nprobing process to exit with a non-zero code and print an error message\nto stderr without a final probe report.
", "displayName": "Output and exit codes from the probed process" }, { "textRaw": "Probing multiple expressions at the same execution point", "name": "probing_multiple_expressions_at_the_same_execution_point", "type": "module", "desc": "When multiple --probe/--expr pairs share the same --probe, the\nexpressions will be evaluated on the same pause in the order they appear\non the command line.
// app.js\nconst x = { x: 42 }; // line 2\nconst y = { y: 35 }; // line 3\nconst z = { ...x, ...y }; // line 4\n$ node inspect --probe app.js:4 --expr 'x' --probe app.js:4 --expr 'y' -- app.js\nPrints
\nHit 1 at app.js:4\n x = {x: 42}\nHit 1 at app.js:4\n y = {y: 35}\nCompleted\n$ node inspect --probe app.js:4 --expr 'x' --probe app.js:4 --expr 'y' --json --preview -- app.js\nPrints
\n{\"v\":1,\"probes\":[{\"expr\":\"x\",\"target\":[\"app.js\",4]},{\"expr\":\"y\",\"target\":[\"app.js\",4]}],\"results\":[{\"probe\":0,\"event\":\"hit\",\"hit\":1,\"result\":{\"type\":\"object\",\"description\":\"Object\",\"preview\":{\"type\":\"object\",\"description\":\"Object\",\"overflow\":false,\"properties\":[{\"name\":\"x\",\"type\":\"number\",\"value\":\"42\"}]}}},{\"probe\":1,\"event\":\"hit\",\"hit\":1,\"result\":{\"type\":\"object\",\"description\":\"Object\",\"preview\":{\"type\":\"object\",\"description\":\"Object\",\"overflow\":false,\"properties\":[{\"name\":\"y\",\"type\":\"number\",\"value\":\"35\"}]}}},{\"event\":\"completed\"}]}\nThe expressions are evaluated in the lexical scope of the probe location when\nexecution reaches it. Avoid probing a variable declared by let or const at its\ndeclaration site, as this leads to a ReferenceError caused by\naccessing the variable in its temporal dead zone (TDZ).
// app.js\nconst x = 42; // line 2\nconsole.log(x); // line 3\n$ node inspect --probe app.js:1 --expr 'x' app.js\nHit 1 at app.js:1\n [error] x = ReferenceError: Cannot access 'x' from debugger\n ...\nCompleted\nInstead, probe at a location where the variable is already initialized:
\n$ node inspect --probe app.js:3 --expr 'x' app.js\nHit 1 at app.js:3\n x = 42\nCompleted\nProbe paths are matched against loaded script URLs by basename, similar to how\nnative debuggers typically match breakpoints. Given:
\nproject/\n - src/utils.js\n - lib/utils.js\n--probe utils.js:10 binds to both files and produces one hit per match.\nTo disambiguate, specify a fuller path that only matches the intended file:
$ node inspect --probe src/utils.js:10 --expr 'x' main.js # matches only src/utils.js\nV8 Inspector integration allows attaching Chrome DevTools to Node.js\ninstances for debugging and profiling. It uses the\nChrome DevTools Protocol.
\nV8 Inspector can be enabled by passing the --inspect flag when starting a\nNode.js application. It is also possible to supply a custom port with that flag,\ne.g. --inspect=9222 will accept DevTools connections on port 9222.
Using the --inspect flag will execute the code immediately before debugger is connected.\nThis means that the code will start running before you can start debugging, which might\nnot be ideal if you want to debug from the very beginning.
In such cases, you have two alternatives:
\n--inspect-wait flag: This flag will wait for debugger to be attached before executing the code.\nThis allows you to start debugging right from the beginning of the execution.--inspect-brk flag: Unlike --inspect, this flag will break on the first line of the code\nas soon as debugger is attached. This is useful when you want to debug the code step by step\nfrom the very beginning, without any code execution prior to debugging.So, when deciding between --inspect, --inspect-wait, and --inspect-brk, consider whether you want\nthe code to start executing immediately, wait for debugger to be attached before execution,\nor break on the first line for step-by-step debugging.
$ node --inspect index.js\nDebugger listening on ws://127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29\nFor help, see: https://nodejs.org/learn/getting-started/debugging\n(In the example above, the UUID dc9010dd-f8b8-4ac5-a510-c1a114ec7d29\nat the end of the URL is generated on the fly, it varies in different\ndebugging sessions.)
", "displayName": "V8 inspector integration for Node.js" } ], "displayName": "Advanced usage" } ] } ] }