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Debugger

Stability: 2 - Stable

V8 comes with an extensive debugger which is accessible out-of-process via a simple TCP protocol. Node.js has a built-in client for this debugger. To use this, start Node.js 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.js'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.

Watchers

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").

Commands 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 isn't loaded yet:

% ./node debug test/fixtures/break-in-module/main.js
< debugger listening on port 5858
connecting to port 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', 23)
Warning: script 'mod.js' was not loaded yet.
  1 var mod = require('./mod.js');
  2 mod.hello();
  3 mod.hello();
debug> c
break in test/fixtures/break-in-module/mod.js:23
 21
 22 exports.hello = function() {
 23   return 'hello from module';
 24 };
 25
debug>

Info

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

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

The V8 debugger can be enabled and accessed either by starting 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 with this it can be connected to with the Node.js debugger. Either connect to the pid or the URI to the debugger. The syntax is:

node debug -p <pid> - Connects to the process via the pid
node debug <URI> - Connects to the process via the URI such as localhost:5858

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