Table of Contents

• Debugger
  • Watchers
  • Command reference
    • Stepping
    • Breakpoints
    • Information
    • Execution control
    • Various
  • Advanced Usage
    • V8 Inspector Integration for Node.js

Debugger#

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#

Breakpoints#

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#

Execution control#

Various#

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 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.

$ 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/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.)