node/doc/api/debugger.markdown

## Debugger

V8 comes with an extensive debugger which is accessible out-of-process via a
simple [TCP protocol](http://code.google.com/p/v8/wiki/DebuggerProtocol).
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.


### 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`.