Node.js

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# Debugger#

Stability: 2 - Stable

Node.js includes an out-of-process debugging utility accessible via a V8 Inspector and built-in debugging client. To use it, start Node.js with the `inspect` argument followed by the path to the script to debug; a prompt will be displayed indicating successful launch of the debugger:

    $ node inspect myscript.js
    < Debugger listening on ws://127.0.0.1:9229/80e7a814-7cd3-49fb-921a-2e02228cd5ba
    < For help, see: https://nodejs.org/en/docs/inspector
    < Debugger attached.
    Break on start in myscript.js:1
    > 1 (function (exports, require, module, __filename, __dirname) { global.x = 5;
      2 setTimeout(() => {
      3   console.log('world');
    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
    global.x = 5;
    setTimeout(() => {
      debugger;
      console.log('world');
    }, 1000);
    console.log('hello');

Once the debugger is run, a breakpoint will occur at line 3:

    $ node inspect myscript.js
    < Debugger listening on ws://127.0.0.1:9229/80e7a814-7cd3-49fb-921a-2e02228cd5ba
    < For help, see: https://nodejs.org/en/docs/inspector
    < Debugger attached.
    Break on start in myscript.js:1
    > 1 (function (exports, require, module, __filename, __dirname) { global.x = 5;
      2 setTimeout(() => {
      3   debugger;
    debug> cont
    < hello
    break in myscript.js:3
      1 (function (exports, require, module, __filename, __dirname) { global.x = 5;
      2 setTimeout(() => {
    > 3   debugger;
      4   console.log('world');
      5 }, 1000);
    debug> next
    break in 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 myscript.js:5
      3   debugger;
      4   console.log('world');
    > 5 }, 1000);
      6 console.log('hello');
      7
    debug> .exit

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 is not loaded yet:

    $ node inspect main.js
    < Debugger listening on ws://127.0.0.1:9229/4e3db158-9791-4274-8909-914f7facf3bd
    < For help, see: https://nodejs.org/en/docs/inspector
    < Debugger attached.
    Break on start in main.js:1
    > 1 (function (exports, require, module, __filename, __dirname) { const mod = require('./mod.js');
      2 mod.hello();
      3 mod.hello();
    debug> setBreakpoint('mod.js', 22)
    Warning: script 'mod.js' was not loaded yet.
    debug> c
    break in mod.js:22
     20 // USE OR OTHER DEALINGS IN THE SOFTWARE.
     21
    >22 exports.hello = function() {
     23   return 'hello from module';
     24 };
    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#

### V8 Inspector Integration for Node.js#

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

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, pass the `--inspect-brk` flag instead of `--inspect`.

    $ node --inspect index.js
    Debugger listening on 127.0.0.1:9229.
    To start debugging, open the following URL in Chrome:
        chrome-devtools://devtools/bundled/js_app.html?experiments=true&v8only=true&ws=127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29

(In the example above, the UUID dc9010dd-f8b8-4ac5-a510-c1a114ec7d29 at the end of the URL is generated on the fly, it varies in different debugging sessions.)

If the Chrome browser is older than 66.0.3345.0, use `inspector.html` instead of `js_app.html` in the above URL.

Chrome DevTools doesn't support debugging Worker Threads yet. ndb can be used to debug them.