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## Debugger
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V8 comes with an extensive debugger which is accessible out-of-process via a
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simple [TCP protocol](http://code.google.com/p/v8/wiki/DebuggerProtocol).
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Node has a built-in client for this debugger. To use this, start Node with the
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`debug` argument; a prompt will appear:
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% node debug myscript.js
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debug>
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At this point `myscript.js` is not yet running. To start the script, enter
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the command `run`. If everything works okay, the output should look like
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this:
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% node debug myscript.js
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debug> run
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debugger listening on port 5858
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connecting...ok
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Node's debugger client doesn't support the full range of commands, but
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simple step and inspection is possible. By putting the statement `debugger;`
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into the source code of your script, you will enable a breakpoint.
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For example, suppose `myscript.js` looked like this:
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// myscript.js
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x = 5;
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setTimeout(function () {
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debugger;
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console.log("world");
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}, 1000);
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console.log("hello");
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Then once the debugger is run, it will break on line 4.
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% ./node debug myscript.js
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debug> run
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debugger listening on port 5858
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connecting...ok
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hello
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break in #<an Object>._onTimeout(), myscript.js:4
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debugger;
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^
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debug> next
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break in #<an Object>._onTimeout(), myscript.js:5
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console.log("world");
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^
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debug> print x
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5
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debug> print 2+2
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4
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debug> next
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world
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break in #<an Object>._onTimeout() returning undefined, myscript.js:6
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}, 1000);
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^
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debug> quit
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A debugging session is active. Quit anyway? (y or n) y
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%
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The `print` command allows you to evaluate variables. The `next` command steps
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over to the next line. There are a few other commands available and more to
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come type `help` to see others.
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### Advanced Usage
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The V8 debugger can be enabled and accessed either by starting Node with
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the `--debug` command-line flag or by signaling an existing Node process
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with `SIGUSR1`.
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