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 does not yet support the full range of commands, 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.
For example, suppose myscript.js
is written as:
// 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 over to the next line. Type help
to see what other commands are available.
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')
.
Commands 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>
Info
-
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 connected to 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 thepid
-
node debug <URI>
- Connects to the process via the URI such as localhost:5858
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