Node Inspector
Overview
Node Inspector is a debugger interface for Node.js applications that uses the Blink Developer Tools (formerly WebKit Web Inspector).
Since version 6.3, Node.js provides a built-in DevTools-based debugger which mostly deprecates Node Inspector, see e.g. this blog post to get started. The built-in debugger is developed directly by the V8/Chromium team and provides certain advanced features (e.g. long/async stack traces) that are too difficult to implement in Node Inspector.
Table of Content
Quick Start
Install
$ npm install -g node-inspector
Start
$ node-debug app.js
where app.js is the name of your main Node application JavaScript file.
See available configuration options here
Debug
The node-debug command will load Node Inspector in your default browser.
NOTE: Node Inspector works in Chrome and Opera only. You have to re-open
the inspector page in one of those browsers if another browser
is your default web browser (e.g. Safari or Internet Explorer).
Node Inspector works almost exactly as the Chrome Developer Tools. Read the
excellent
DevTools overview
to get started.
Other useful resources:
Features
The Blink DevTools debugger is a powerful JavaScript debugger interface.
Node Inspector supports almost all of the debugging features of DevTools, including:
- Navigate in your source files
- Set breakpoints (and specify trigger conditions)
- Step over, step in, step out, resume (continue)
- Inspect scopes, variables, object properties
- Hover your mouse over an expression in your source to display its value in
a tooltip
- Edit variables and object properties
- Continue to location
- Break on exceptions
- Disable/enable all breakpoints
- CPU and HEAP profiling
- Network client requests inspection
- Console output inspection
Cool stuff
- Node Inspector uses WebSockets, so no polling for breaks.
- Remote debugging and debugging remote machine.
- Live edit of running code,
optionally persisting changes back to the file-system.
- Set breakpoints in files that are not loaded into V8 yet - useful for
debugging module loading/initialization.
- Embeddable in other applications - see Embedding HOWTO
for more details.
Known Issues
- Be careful about viewing the contents of Buffer objects,
each byte is displayed as an individual array element;
for most Buffers this will take too long to render.
- While not stopped at a breakpoint the console doesn't always
behave as you might expect. See the
issue #146.
- Break on uncaught exceptions does not work in all Node versions,
you need at least v0.11.3 (see
node#5713).
- Debugging multiple processes (e.g. cluster) is cumbersome.
Read the following blog post for instructions:
Debugging Clustered Apps with Node-Inspector
Troubleshooting
My script runs too fast to attach the debugger.
The debugged process must be started with --debug-brk, this way the script is paused on the first line.
Note: node-debug adds this option for you by default.
I got the UI in a weird state.
When in doubt, refresh the page in browser
Can I debug remotely?
Yes. Node Inspector must be running on the same machine, but your browser can be anywhere.
Just make sure port 8080 is accessible.
And if Node Inspector is not running on your remote machine, you can also debug it as long as your local machine can connect it.
In this way, you must launch Node Inspector with --no-inject which means some features are not supported such as profiling and consoling output inspection.
So how to debug remote machine with your local Node Inspector?
option 1
$ node-inspector --debug-host 192.168.0.2 --no-inject
then open the url
http://127.0.0.1:8080/debug?port=5858
option 2
$ node-inspector --no-inject
then specify the remote machine address as a host parameter in the url
e.g.) http://127.0.0.1:8080/debug?host=192.168.123.12&port=5858
How do I specify files to hide?
Create a JSON-encoded array. You must escape quote characters when using a command-line option.
$ node-inspector --hidden='["node_modules/framework"]'
Note that the array items are interpreted as regular expressions.
UI doesn't load or doesn't work and refresh didn't help
Make sure that you have adblock disabled as well as any other content blocking scripts and plugins.
You may want to delete debug session metadata if for example Node Inspector gets in a bad state with some
watch variables that were function calls (possibly into some special c-bindings). In such cases, even restarting
the application/debug session may not fix the problem.
Node Inspector stores debug session metadata in the HTML5 local storage.
You can inspect the contents of local storage and remove any items as
needed. In Google Chrome, you can execute any of the following in the JavaScript console:
// Remove all
window.localStorage.clear()
// Or, to list keys so you can selectively remove them with removeItem()
window.localStorage
// Remove all the watch expressions
window.localStorage.removeItem('watchExpressions')
// Remove all the breakpoints
window.localStorage.removeItem('breakpoints')
When you are done cleaning up, hit refresh in the browser.
Node Inspector takes a long time to start up.
Try setting --no-preload to true. This option disables searching disk for *.js at startup.
Code will still be loaded into Node Inspector at runtime, as modules are required.
How do I debug Mocha unit-tests?
You have to start _mocha as the debugged process and make sure
the execution pauses on the first line. This way you have enough
time to set your breakpoints before the tests are run.
$ node-debug _mocha
How do I debug Gulp tasks?
If you are running on a Unix system you can simply run the following command.
The $(which ..) statement gets replaced with the full path to the gulp-cli.
$ node-debug $(which gulp) task
If you are running on Windows, you have to get the full path of gulp.js
to make an equivalent command:
> node-debug %appdata%\npm\node_modules\gulp\bin\gulp.js task
You can omit the task part to run the default task.
Advanced Use
While running node-debug is a convenient way to start your debugging
session, there may come time when you need to tweak the default setup.
There are three steps needed to get you up and debugging:
1. Start the Node Inspector server
$ node-inspector
You can leave the server running in background, it's possible to debug
multiple processes using the same server instance.
2. Enable debug mode in your Node process
You can either start Node with a debug flag like:
$ node --debug your/node/program.js
or, to pause your script on the first line:
$ node --debug-brk your/short/node/script.js
Or you can enable debugging on a node that is already running by sending
it a signal:
Get the PID of the node process using your favorite method.
pgrep or ps -ef are good
$ pgrep -l node
2345 node your/node/server.js
Send it the USR1 signal
$ kill -s USR1 2345
Windows
Windows does not support UNIX signals. To enable debugging, you can use
an undocumented API function process._debugProcess(pid):
Get the PID of the node process using your favorite method, e.g.
> tasklist /FI "IMAGENAME eq node.exe"
Image Name PID Session Name Session# Mem Usage
========================= ======== ================ =========== ============
node.exe 3084 Console 1 11,964 K
Call the API:
> node -e "process._debugProcess(3084)"
3. Load the debugger UI
Open http://127.0.0.1:8080/?port=5858 in the Chrome browser.
Configuration
Both node-inspector and node-debug use rc module
to manage configuration options.
Places for configuration:
- command line arguments (parsed by yargs)
- environment variables prefixed with
node-inspector_
- if you passed an option
--config file then from that file
- a local
.node-inspectorrc or the first found looking in ./ ../ ../../ ../../../ etc.
$HOME/.node-inspectorrc$HOME/.node-inspector/config$HOME/.config/node-inspector$HOME/.config/node-inspector/config/etc/node-inspectorrc/etc/node-inspector/config
All configuration sources that where found will be flattened into one object,
so that sources earlier in this list override later ones.
Options
| Option |
Alias |
Default |
Description |
| general |
|
|
|
| --help |
-h |
|
Display information about available options.
Use --help -l to display full usage info.
Use --help <option> to display quick help on option. |
| --version |
-v |
|
Display Node Inspector's version. |
| --debug-port |
-d |
5858 |
Node/V8 debugger port.
(node --debug={port}) |
| --web-host |
|
0.0.0.0 |
Host to listen on for Node Inspector's web interface.
node-debug listens on 127.0.0.1 by default. |
| --web-port |
-p |
8080 |
Port to listen on for Node Inspector's web interface. |
| node-debug |
|
|
|
| --debug-brk |
-b |
true |
Break on the first line.
(node --debug-brk) |
| --nodejs |
|
[] |
Pass NodeJS options to debugged process.
(node --option={value}) |
| --script |
|
[] |
Pass options to debugged process.
(node app --option={value}) |
| --cli |
-c |
false |
CLI mode, do not open browser. |
| node-inspector |
|
|
|
| --save-live-edit |
|
false |
Save live edit changes to disk (update the edited files). |
| --preload |
|
true |
Preload *.js files. You can disable this option
to speed up the startup. |
| --inject |
|
true |
Enable injection of debugger extensions into the debugged process. It's possible disable only part of injections using subkeys --no-inject.network. Allowed keys : network, profiles, console. |
| --hidden |
|
[] |
Array of files to hide from the UI,
breakpoints in these files will be ignored.
All paths are interpreted as regular expressions. |
| --stack-trace-limit |
|
50 |
Number of stack frames to show on a breakpoint. |
| --ssl-key |
|
|
Path to file containing a valid SSL key. |
| --ssl-cert |
|
|
Path to file containing a valid SSL certificate. |
Usage examples
Command line
$ node-debug [general-options] [node-debug-options] [node-inspector-options] [script]
$ node-inspector [general-options] [node-inspector-options]
Usage
Display full usage info:
$ node-debug --help -l
Set debug port of debugging process to 5859:
$ node-debug -p 5859 app
Pass --web-host=127.0.0.2 to node-inspector. Start node-inspector to listen on 127.0.0.2:
$ node-debug --web-host 127.0.0.2 app
Pass --option=value to debugging process:
$ node-debug app --option value
Start node-inspector to listen on HTTPS:
$ node-debug --ssl-key ./ssl/key.pem --ssl-cert ./ssl/cert.pem app
Ignore breakpoints in files stored in node_modules folder or ending in .test.js:
$ node-debug --hidden node_modules/ --hidden \.test\.js$ app
Add --harmony flag to the node process running the debugged script:
$ node-debug --nodejs --harmony app
Disable preloading of .js files:
$ node-debug --no-preload app
RC Configuration
Use dashed option names in RC files. Sample config file (to be saved as .node-inspectorrc):
{
"web-port": 8088,
"web-host": "0.0.0.0",
"debug-port": 5858,
"save-live-edit": true,
"preload": false,
"hidden": ["\.test\.js$", "node_modules/"],
"nodejs": ["--harmony"],
"stack-trace-limit": 50,
"ssl-key": "./ssl/key.pem",
"ssl-cert": "./ssl/cert.pem"
}
Contributing Code
Making Node Inspector the best debugger for node.js cannot be achieved without
the help of the community. The following resources should help you to get
started.
Credits
Current maintainers
Alumni
- Danny Coates - the original author and a sole maintainer for several years.
- Miroslav Bajtoš - sponsored by StrongLoop, maintained Node Inspector through the Node.js 0.10 era.
- 3y3 - maintained Node Inspector in 2015-2016
Contributors
Big thanks to the many contributors to the project, see Contributors on GitHub
A debugger interface for Node.js applications that uses the Blink Developer Tools (formerly WebKit Web Inspector) — essentially the Chrome DevTools UI pointed at your Node code.