Debugger
Debugger
Node.js includes an out-of-process debugging utility accessible via a 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 127.0.0.1:5858 connecting to 127.0.0.1:5858 ... ok break in /home/indutny/Code/git/indutny/myscript.js:1 > 1 global.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 global.x = 5; setTimeout(() => { debugger; console.log('world'); }, 1000); console.log('hello');
Once the debugger is run, a breakpoint will occur at line 3:
$ node debug myscript.js < Debugger listening on 127.0.0.1:5858 connecting to 127.0.0.1:5858 ... ok break in /home/indutny/Code/git/indutny/myscript.js:1 > 1 global.x = 5; 2 setTimeout(() => { 3 debugger; debug> cont < hello break in /home/indutny/Code/git/indutny/myscript.js:3 1 global.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 break in /home/indutny/Code/git/indutny/myscript.js:5 < world 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 is not loaded yet:
$ node debug test/fixtures/break-in-module/main.js < Debugger listening on 127.0.0.1:5858 connecting to 127.0.0.1:5858 ... ok break in test/fixtures/break-in-module/main.js:1 > 1 const mod = require('./mod.js'); 2 mod.hello(); 3 mod.hello(); debug> setBreakpoint('mod.js', 2) Warning: script 'mod.js' was not loaded yet. > 1 const mod = require('./mod.js'); 2 mod.hello(); 3 mod.hello(); 4 debugger; 5 6 }); debug> c break in test/fixtures/break-in-module/mod.js:2 1 exports.hello = function() { > 2 return 'hello from module'; 3 }; 4 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
V8 Inspector Integration for Node.js
V8 Inspector integration allows attaching Chrome DevTools to Node.js instances for debugging and profiling. It uses the Chrome Debugging 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.
To break on the first line of the application code, pass the --inspect-brk
flag instead of --inspect
.
$ node --inspect index.js Debugger listening on 127.0.0.1:9229. To start debugging, open the following URL in Chrome: chrome-devtools://devtools/bundled/inspector.html?experiments=true&v8only=true&ws=127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29
(In the example above, the UUID dc9010dd-f8b8-4ac5-a510-c1a114ec7d29 at the end of the URL is generated on the fly, it varies in different debugging sessions.)
© Joyent, Inc. and other Node contributors
Licensed under the MIT License.
Node.js is a trademark of Joyent, Inc. and is used with its permission.
We are not endorsed by or affiliated with Joyent.
https://nodejs.org/dist/latest-v8.x/docs/api/debugger.html