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Stability: 2 - Stable
Node.js includes a command-line debugging utility. The Node.js debugger client is not a full-featured debugger, but simple stepping and inspection are possible.

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Starting the Debugger

To use it, start Node.js with the inspect argument followed by the path to the script to debug: 
node inspect myscript.js
This will output something like: 
< Debugger listening on ws://127.0.0.1:9229/621111f9-ffcb-4e82-b718-48a145fa5db8
< For help, see: https://nodejs.org/en/docs/inspector
<
connecting to 127.0.0.1:9229 ... ok
< Debugger attached.
<
 ok
Break on start in myscript.js:2
  1 // myscript.js
> 2 global.x = 5;
  3 setTimeout(() => {
  4   debugger;
debug>
The debugger automatically breaks on the first executable line. To instead run until the first breakpoint (specified by a debugger statement), set the NODE_INSPECT_RESUME_ON_START environment variable to 1.

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Debugger Commands

Type help to see available commands. Pressing enter without typing a command will repeat the previous debugger command.

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Stepping

  • cont, c - Continue execution
  • next, n - Step next
  • step, s - Step in
  • out, o - Step out
  • pause - Pause running code (like pause button in Developer Tools)

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Breakpoints

  • setBreakpoint(), sb() - Set breakpoint on current line
  • setBreakpoint(line), sb(line) - Set breakpoint on specific line
  • setBreakpoint('fn()'), sb(...) - Set breakpoint on a first statement in function’s body
  • setBreakpoint('script.js', 1), sb(...) - Set breakpoint on first line of script.js
  • setBreakpoint('script.js', 1, 'num < 4'), sb(...) - Set conditional breakpoint on first line of script.js that only breaks when num < 4 evaluates to true
  • clearBreakpoint('script.js', 1), cb(...) - Clear breakpoint in script.js on line 1

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Information

  • backtrace, bt - Print backtrace of current execution frame
  • list(5) - List scripts source code with 5 line context (5 lines before and after)
  • watch(expr) - Add expression to watch list
  • unwatch(expr) - Remove expression from watch list
  • unwatch(index) - Remove expression at specific index from watch list
  • watchers - List all watchers and their values (automatically listed on each breakpoint)
  • repl - Open debugger’s repl for evaluation in debugging script’s context
  • exec expr, p expr - Execute an expression in debugging script’s context and print its value

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Execution Control

  • run - Run script (automatically runs on debugger’s start)
  • restart - Restart script
  • kill - Kill script

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Various

  • scripts - List all loaded scripts
  • version - Display V8’s version

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Watchers

It is possible to watch expression and variable values while debugging. On every breakpoint, each expression from the watchers list will be evaluated in the current context and displayed immediately before the breakpoint’s source code listing. To begin watching an expression, type watch('my_expression'). The command watchers will print the active watchers. To remove a watcher, type unwatch('my_expression').

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Example Session

$ node inspect myscript.js
< Debugger listening on ws://127.0.0.1:9229/f1ed133e-7876-495b-83ae-c32c6fc319c2
< For help, see: https://nodejs.org/en/docs/inspector
<
connecting to 127.0.0.1:9229 ... ok
< Debugger attached.
<
< hello
<
break in myscript.js:4
  2 global.x = 5;
  3 setTimeout(() => {
> 4   debugger;
  5   console.log('world');
  6 }, 1000);
debug> next
break in myscript.js:5
  3 setTimeout(() => {
  4   debugger;
> 5   console.log('world');
  6 }, 1000);
  7 console.log('hello');
debug> repl
Press Ctrl+C to leave debug repl
> x
5
> 2 + 2
4
debug> next
< world
<
break in myscript.js:6
  4   debugger;
  5   console.log('world');
> 6 }, 1000);
  7 console.log('hello');
  8
debug> .exit
$
The repl command allows code to be evaluated remotely. The next command steps to the next line.

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Setting Breakpoints in Files Not Yet Loaded

It is possible to set a breakpoint in a file (module) that is not loaded yet: 
$ node inspect main.js
< Debugger listening on ws://127.0.0.1:9229/48a5b28a-550c-471b-b5e1-d13dd7165df9
< For help, see: https://nodejs.org/en/docs/inspector
<
connecting to 127.0.0.1:9229 ... ok
< Debugger attached.
<
Break on start in main.js:1
> 1 const mod = require('./mod.js');
  2 mod.hello();
  3 mod.hello();
debug> setBreakpoint('mod.js', 22)
Warning: script 'mod.js' was not loaded yet.
debug> c
break in mod.js:22
 20 // USE OR OTHER DEALINGS IN THE SOFTWARE.
 21
>22 exports.hello = function() {
 23   return 'hello from module';
 24 };
debug>

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Conditional Breakpoints

It is possible to set a conditional breakpoint that only breaks when a given expression evaluates to true: 
$ node inspect main.js
< Debugger listening on ws://127.0.0.1:9229/ce24daa8-3816-44d4-b8ab-8273c8a66d35
< For help, see: https://nodejs.org/en/docs/inspector
<
connecting to 127.0.0.1:9229 ... ok
< Debugger attached.
Break on start in main.js:7
  5 }
  6
> 7 addOne(10);
  8 addOne(-1);
  9
debug> setBreakpoint('main.js', 4, 'num < 0')
  1 'use strict';
  2
  3 function addOne(num) {
> 4   return num + 1;
  5 }
  6
  7 addOne(10);
  8 addOne(-1);
  9
debug> cont
break in main.js:4
  2
  3 function addOne(num) {
> 4   return num + 1;
  5 }
  6
debug> exec('num')
-1
debug>

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V8 Inspector Integration

V8 Inspector integration allows attaching Chrome DevTools to Node.js instances for debugging and profiling. It uses the Chrome DevTools Protocol. V8 Inspector can be enabled by passing the --inspect flag when starting a Node.js application. It is also possible to supply a custom port with that flag, e.g. --inspect=9222 will accept DevTools connections on port 9222.

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Inspector Flags

  • --inspect - Activate inspector on host:port. Default is 127.0.0.1:9229. The code will execute immediately before debugger is connected
  • --inspect-wait - Wait for debugger to be attached before executing the code. This allows you to start debugging right from the beginning of the execution
  • --inspect-brk - Break on the first line of the code as soon as debugger is attached. This is useful when you want to debug the code step by step from the very beginning

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Example

node --inspect index.js
This will output: 
Debugger listening on ws://127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29
For help, see: https://nodejs.org/en/docs/inspector
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.

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Using Chrome DevTools

Once the inspector is enabled, you can connect Chrome DevTools:
  1. Open Chrome browser
  2. Navigate to chrome://inspect
  3. Click “Configure…” and ensure your target host and port are listed
  4. Your Node.js application should appear under “Remote Target”
  5. Click “inspect” to open DevTools
Chrome DevTools provides a full-featured debugging experience with:
  • Breakpoint debugging
  • Step through code execution
  • Inspect variables and scope
  • Console evaluation
  • CPU and memory profiling
  • Network inspection (with appropriate flags)