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# Debugger#

    Stability: 2 - Stable

Node.js includes a full-featured out-of-process debugging utility accessible via a simple TCP-based protocol and built-in debugging client. To use it, start Node.js with the `debug` argument followed by the path to the script to debug; a prompt will be displayed indicating successful launch of the debugger:

    $ 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(() => {
      3   debugger;
    debug>

Node.js's debugger client is not a full-featured debugger, but simple step and inspection are possible.

Inserting the statement `debugger;` into the source code of a script will enable a breakpoint at that position in the code:

    // myscript.js
    x = 5;
    setTimeout(() => {
      debugger;
      console.log('world');
    }, 1000);
    console.log('hello');

Once the debugger is run, a breakpoint will occur at 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(() => {
      3   debugger;
    debug> cont
    < hello
    break in /home/indutny/Code/git/indutny/myscript.js:3
      1 x = 5;
      2 setTimeout(() => {
      3   debugger;
      4   console.log('world');
      5 }, 1000);
    debug> next
    break in /home/indutny/Code/git/indutny/myscript.js:4
      2 setTimeout(() => {
      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 code to be evaluated remotely. The `next` command steps to the next line. Type `help` to see what other commands are available.

Pressing `enter` without typing a command will repeat the previous debugger command.

## Watchers#

It is possible to watch expression and variable values while debugging. On every breakpoint, each expression from the watchers list will be evaluated in the current context and displayed immediately before the breakpoint's source code listing.

To begin watching an expression, type `watch('my_expression')`. The command `watchers` will print the active watchers. To remove a watcher, type `unwatch('my_expression')`.

## Command 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 = () => {
     23   return 'hello from module';
     24 };
     25
    debug>

### Information#

- `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
- `exec expr` - Execute an expression 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#

An alternative way of enabling and accessing the debugger is to start 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 this way, it can be inspected using the Node.js debugger by either connecting to the `pid` of the running process or via URI reference to the listening debugger:

- `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

## V8 Inspector Integration for Node.js#

NOTE: This is an experimental feature.

V8 Inspector integration allows attaching Chrome DevTools to Node.js instances for debugging and profiling.

V8 Inspector can be enabled by passing the `--inspect` flag when starting a Node.js application. It is also possible to supply a custom port with that flag, e.g. `--inspect=9222` will accept DevTools connections on port 9222.

To break on the first line of the application code, provide the `--debug-brk` flag in addition to `--inspect`.