Node.js v7.2.0 Documentation


Debugger#

Stability: 2 - Stable

Node.js includes a full-featured out-of-process debugging utility accessible via a simple TCP-based protocol and built-in debugging client. To use it, start Node.js with the debug argument followed by the path to the script to debug; a prompt will be displayed indicating successful launch of the debugger:

$ node debug myscript.js
< debugger listening on port 5858
connecting... ok
break in /home/indutny/Code/git/indutny/myscript.js:1
  1 x = 5;
  2 setTimeout(() => {
  3   debugger;
debug>

Node.js's debugger client is not a full-featured debugger, but simple step and inspection are possible.

Inserting the statement debugger; into the source code of a script will enable a breakpoint at that position in the code:

// myscript.js
x = 5;
setTimeout(() => {
  debugger;
  console.log('world');
}, 1000);
console.log('hello');

Once the debugger is run, a breakpoint will occur at line 4:

$ node debug myscript.js
< debugger listening on port 5858
connecting... ok
break in /home/indutny/Code/git/indutny/myscript.js:1
  1 x = 5;
  2 setTimeout(() => {
  3   debugger;
debug> cont
< hello
break in /home/indutny/Code/git/indutny/myscript.js:3
  1 x = 5;
  2 setTimeout(() => {
  3   debugger;
  4   console.log('world');
  5 }, 1000);
debug> next
break in /home/indutny/Code/git/indutny/myscript.js:4
  2 setTimeout(() => {
  3   debugger;
  4   console.log('world');
  5 }, 1000);
  6 console.log('hello');
debug> repl
Press Ctrl + C to leave debug repl
> x
5
> 2+2
4
debug> next
< world
break in /home/indutny/Code/git/indutny/myscript.js:5
  3   debugger;
  4   console.log('world');
  5 }, 1000);
  6 console.log('hello');
  7
debug> quit

The repl command allows code to be evaluated remotely. The next command steps to the next line. Type help to see what other commands are available.

Pressing enter without typing a command will repeat the previous debugger command.

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').

Command reference#

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)

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 functions body
  • setBreakpoint('script.js', 1), sb(...) - Set breakpoint on first line of script.js
  • clearBreakpoint('script.js', 1), cb(...) - Clear breakpoint in script.js on line 1

It is also possible to set a breakpoint in a file (module) that isn't loaded yet:

$ node debug test/fixtures/break-in-module/main.js
< debugger listening on port 5858
connecting to port 5858... ok
break in test/fixtures/break-in-module/main.js:1
  1 var mod = require('./mod.js');
  2 mod.hello();
  3 mod.hello();
debug> setBreakpoint('mod.js', 23)
Warning: script 'mod.js' was not loaded yet.
  1 var mod = require('./mod.js');
  2 mod.hello();
  3 mod.hello();
debug> c
break in test/fixtures/break-in-module/mod.js:23
 21
 22 exports.hello = () => {
 23   return 'hello from module';
 24 };
 25
debug>

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
  • 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 - Execute an expression in debugging script's context

Execution control#

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

Various#

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

Advanced Usage#

An alternative way of enabling and accessing the debugger is to start Node.js with the --debug command-line flag or by signaling an existing Node.js process with SIGUSR1.

Once a process has been set in debug mode this way, it can be inspected using the Node.js debugger by either connecting to the pid of the running process or via URI reference to the listening debugger:

  • node debug -p <pid> - Connects to the process via the pid
  • node debug <URI> - Connects to the process via the URI such as localhost:5858

V8 Inspector Integration for Node.js#

NOTE: This is an experimental feature.

V8 Inspector integration allows attaching Chrome DevTools to Node.js instances for debugging and profiling.

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.

To break on the first line of the application code, provide the --debug-brk flag in addition to --inspect.

$ node --inspect index.js
Debugger listening on port 9229.
Warning: This is an experimental feature and could change at any time.
To start debugging, open the following URL in Chrome:
    chrome-devtools:[email protected]84980/inspector.html?experiments=true&v8only=true&ws=localhost:9229/node