# Debugger > Stability: 2 - Stable Node.js includes an out-of-process debugging utility accessible via a [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: ```txt $ node debug myscript.js < Debugger listening on 127.0.0.1:5858 connecting to 127.0.0.1:5858 ... ok break in /home/indutny/Code/git/indutny/myscript.js:1 > 1 global.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: ```js // 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: ```txt $ node debug myscript.js < Debugger listening on 127.0.0.1:5858 connecting to 127.0.0.1:5858 ... ok break in /home/indutny/Code/git/indutny/myscript.js:1 > 1 global.x = 5; 2 setTimeout(() => { 3 debugger; debug> cont < hello break in /home/indutny/Code/git/indutny/myscript.js:3 1 global.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 break in /home/indutny/Code/git/indutny/myscript.js:5 < world 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 is not loaded yet: ```txt $ node debug test/fixtures/break-in-module/main.js < Debugger listening on 127.0.0.1:5858 connecting to 127.0.0.1:5858 ... ok break in test/fixtures/break-in-module/main.js:1 > 1 const mod = require('./mod.js'); 2 mod.hello(); 3 mod.hello(); debug> setBreakpoint('mod.js', 2) Warning: script 'mod.js' was not loaded yet. > 1 const mod = require('./mod.js'); 2 mod.hello(); 3 mod.hello(); 4 debugger; 5 6 }); debug> c break in test/fixtures/break-in-module/mod.js:2 1 exports.hello = function() { > 2 return 'hello from module'; 3 }; 4 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 ### TCP-based protocol > Stability: 0 - Deprecated: Use [V8 Inspector Integration][] instead. The debug protocol used by the `--debug` flag was removed from V8. 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 ` - Connects to the process via the `pid` * `node debug ` - 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. It uses the [Chrome Debugging 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`. ```txt $ node --inspect index.js Debugger listening on port 9229. Warning: This is an experimental feature and could change at any time. To start debugging, open the following URL in Chrome: chrome-devtools://devtools/bundled/inspector.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.) [Chrome Debugging Protocol]: https://chromedevtools.github.io/debugger-protocol-viewer/ [TCP-based protocol]: #debugger_tcp_based_protocol [V8 Inspector Integration]: #debugger_v8_inspector_integration_for_node_js