V8 comes with an extensive debugger which is accessible out-of-process via a
simple TCP protocol.
Node has a built-in client for this debugger. To use this, start Node with the
debug
argument; a prompt will appear:
% 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(function () {
3 debugger;
debug>
Node's debugger client doesn't support the full range of commands, but
simple step and inspection is possible. By putting the statement debugger;
into the source code of your script, you will enable a breakpoint.
For example, suppose myscript.js
looked like this:
// myscript.js
x = 5;
setTimeout(function () {
debugger;
console.log("world");
}, 1000);
console.log("hello");
Then once the debugger is run, it will break on 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(function () {
3 debugger;
debug> cont
< hello
break in /home/indutny/Code/git/indutny/myscript.js:3
1 x = 5;
2 setTimeout(function () {
3 debugger;
4 console.log("world");
5 }, 1000);
debug> next
break in /home/indutny/Code/git/indutny/myscript.js:4
2 setTimeout(function () {
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 you to evaluate code remotely. The next
command
steps over to the next line. There are a few other commands available and more
to come. Type help
to see others.
You can watch expression and variable values while debugging your code. On every breakpoint each expression from the watchers list will be evaluated in the current context and displayed just before the breakpoint's source code listing.
To start watching an expression, type watch("my_expression")
. watchers
prints the active watchers. To remove a watcher, type
unwatch("my_expression")
.
cont
, c
- Continue executionnext
, n
- Step nextstep
, s
- Step inout
, o
- Step outpause
- Pause running code (like pause button in Developer TOols)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
, cb(...)
- Clear breakpointbacktrace
, 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 contextrun
- Run script (automatically runs on debugger's start)restart
- Restart scriptkill
- Kill scriptscripts
- List all loaded scriptsversion
- Display v8's versionThe V8 debugger can be enabled and accessed either by starting Node with
the --debug
command-line flag or by signaling an existing Node process
with SIGUSR1
.