Debugger#
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.
To use it, start Node.js with the inspect
argument followed by the path to the
script to debug.
$ node inspect myscript.js
< Debugger listening on ws://127.0.0.1:9229/621111f9-ffcb-4e82-b718-48a145fa5db8
< For help, see: https://nodejs.org/en/docs/inspector
<
< 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
.
$ cat myscript.js
// myscript.js
global.x = 5;
setTimeout(() => {
debugger;
console.log('world');
}, 1000);
console.log('hello');
$ NODE_INSPECT_RESUME_ON_START=1 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. 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
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
It is also 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
<
< Debugger attached.
<
ok
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>
It is also 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
< 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>
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
, p expr
: Execute an expression in debugging script's context and
print its value
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 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.
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 ws://127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29
For help, see: https://nodejs.org/en/docs/inspector
(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.)
If the Chrome browser is older than 66.0.3345.0,
use inspector.html
instead of js_app.html
in the above URL.
Chrome DevTools doesn't support debugging worker threads yet.
ndb can be used to debug them.