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
• Features
• Known Issues
• Troubleshooting
• Advanced Use
• Configuration
• Contributing
• Credits

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:

• Documentation specific to Node Inspector provided by StrongLoop: Debugging with Node Inspector
• Miroslav's talk How to Debug Node Apps with Node Inspector
• Danny's screencasts (most likely outdated by now)
• Getting Started from scratch on wiki (most likely outdated by now)

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.

How can I (selectively) delete debug session metadata?

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:

1. 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

2. 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):

1. 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

2. 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

Usage examples

Command line

Format

$ 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.

• Contributing
• Developer's Guide
• Easy Picks

Credits

Current maintainers

• 淘小杰 (hustxiaoc)

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