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Node.js v14.15.5-rc.0 Documentation
Table of Contents
Debugger#
Node.js includes an out-of-process debugging utility accessible via a
V8 Inspector and built-in debugging client. To use it, start Node.js
with the inspect
argument followed by the path to the script to debug; a
prompt will be displayed indicating successful launch of the debugger:
$ node inspect myscript.js
< Debugger listening on ws://127.0.0.1:9229/80e7a814-7cd3-49fb-921a-2e02228cd5ba
< For help, see: https://nodejs.org/en/docs/inspector
< Debugger attached.
Break on start in myscript.js:1
> 1 (function (exports, require, module, __filename, __dirname) { global.x = 5;
2 setTimeout(() => {
3 console.log('world');
debug>
The Node.js 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 inspect myscript.js
< Debugger listening on ws://127.0.0.1:9229/80e7a814-7cd3-49fb-921a-2e02228cd5ba
< For help, see: https://nodejs.org/en/docs/inspector
< Debugger attached.
Break on start in myscript.js:1
> 1 (function (exports, require, module, __filename, __dirname) { global.x = 5;
2 setTimeout(() => {
3 debugger;
debug> cont
< hello
break in myscript.js:3
1 (function (exports, require, module, __filename, __dirname) { global.x = 5;
2 setTimeout(() => {
> 3 debugger;
4 console.log('world');
5 }, 1000);
debug> next
break in 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 myscript.js:5
3 debugger;
4 console.log('world');
> 5 }, 1000);
6 console.log('hello');
7
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 executionnext
,n
: Step nextstep
,s
: Step inout
,o
: Step outpause
: Pause running code (like pause button in Developer Tools)
Breakpoints#
setBreakpoint()
,sb()
: Set breakpoint on current linesetBreakpoint(line)
,sb(line)
: Set breakpoint on specific linesetBreakpoint('fn()')
,sb(...)
: Set breakpoint on a first statement in function's bodysetBreakpoint('script.js', 1)
,sb(...)
: Set breakpoint on first line ofscript.js
setBreakpoint('script.js', 1, 'num < 4')
,sb(...)
: Set conditional breakpoint on first line ofscript.js
that only breaks whennum < 4
evaluates totrue
clearBreakpoint('script.js', 1)
,cb(...)
: Clear breakpoint inscript.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://127.0.0.1:9229/4e3db158-9791-4274-8909-914f7facf3bd
< For help, see: https://nodejs.org/en/docs/inspector
< Debugger attached.
Break on start in main.js:1
> 1 (function (exports, require, module, __filename, __dirname) { 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 framelist(5)
: List scripts source code with 5 line context (5 lines before and after)watch(expr)
: Add expression to watch listunwatch(expr)
: Remove expression from watch listwatchers
: List all watchers and their values (automatically listed on each breakpoint)repl
: Open debugger's repl for evaluation in debugging script's contextexec expr
: Execute an expression in debugging script's context
Execution control#
run
: Run script (automatically runs on debugger's start)restart
: Restart scriptkill
: Kill script
Various#
scripts
: List all loaded scriptsversion
: 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.