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Node.js v6.17.1 Documentation
Table of Contents
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 [::]: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 [::]: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 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 functions bodysetBreakpoint('script.js', 1)
,sb(...)
- Set breakpoint on first line of script.jsclearBreakpoint('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 [::]:5858
connecting to 127.0.0.1:5858 ... 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', 2)
Warning: script 'mod.js' was not loaded yet.
> 1 var 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 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#
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 thepid
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://devtools/remote/serve_file/@60cd6e859b9f557d2312f5bf532f6aec5f284980/inspector.html?experiments=true&v8only=true&ws=127.0.0.1:9229/3a6d0a9e-0707-48f8-a7c6-48f157b67ab5
(In the example above, the UUID 3a6d0a9e-0707-48f8-a7c6-48f157b67ab5 at the end of the URL is generated on the fly, it varies in different debugging sessions.)