1.9 KiB
Debugger
V8 comes with an extensive debugger which is accessable 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.