node/doc/api/debugger.markdown

Debugger

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
debug>

At this point myscript.js is not yet running. To start the script, enter the command run. If everything works okay, the output should look like this:

% node debug myscript.js
debug> run
debugger listening on port 5858
connecting...ok

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
debug> run
debugger listening on port 5858
connecting...ok
hello
break in #<an Object>._onTimeout(), myscript.js:4
  debugger;
  ^
debug> next
break in #<an Object>._onTimeout(), myscript.js:5
  console.log("world");
  ^
debug> print x
5
debug> print 2+2
4
debug> next
world
break in #<an Object>._onTimeout() returning undefined, myscript.js:6
}, 1000);
^
debug> quit
A debugging session is active. Quit anyway? (y or n) y
%

The print command allows you to evaluate variables. 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.

Advanced Usage

The 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.