Node Inspector

Overview

Node Inspector is a debugger interface for Node.js applications that uses the Blink Developer Tools (formerly WebKit Web Inspector).

The project maintenance and support is sponsored by StrongLoop.

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.

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: Running 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• Danny's screencasts (most likely outdated by now)
• Getting Started from scratch on wiki (most likely outdated by now)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

Cool stuff

• Node Inspector uses WebSockets, so no polling for breaks.
• Remote debugging
• 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.
• Javascript from top to bottom :)
• 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.
• Profiler is not implemented yet. Have a look at node-webkit-agent in the meantime.
• 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.

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.

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

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/debug?port=5858 in the Chrome browser.

Configuration

node-debug

Command line options:

--debug-brk, -b         Break on the first line (`node --debug-brk`) [default: true]
--web-port, -p, --port  Node Inspector port (`node-inspector --web-port={port}`)
--debug-port, -d        Node/V8 debugger port (`node --debug={port}`)
--cli, -c               CLI mode, do not open browser.
--version, -v           Print Node Inspector's version.
--help, -h              Show this help.

node-inspector

node-inspector uses rc [github] module to collect options.

Places for configuration:

• command line arguments (parsed by optimist)
• enviroment 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
• options from config.json for backward compatibility
• defaults described in Node Inspector`s ./lib/config.js.

All configuration sources that where found will be flattened into one object, so that sources earlier in this list override later ones.

Use dashed option names in RC files. Sample config file:

{
  "web-port": 8088,
  "web-host": null,
  "debug-port": 5858,
  "save-live-edit": true,
  "no-preload": true,
  "hidden": [],
  "stack-trace-limit": 50
}

List of predefined options:

       Option            Default                  Description
--help               |             | Print information about options
--web-port           |    8080     | Port to host the inspector
--web-host           |  127.0.0.1  | Host to listen on
--debug-port         |    5858     | Port to connect to the debugging app
--save-live-edit     |    false    | Save live edit changes to disk
                     |             |   (update the edited files)
--no-preload         |    false    | Disables preloading *.js to speed up startup
--hidden             |     []      | Array of files to hide from the UI
                     |             |   (breakpoints in these files will be ignored)
--stack-trace-limit  |     50      | Number of stack frames to show on a breakpoint

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

Maintainers

• Danny Coates - the original author and a sole maintainer for several years.
• Miroslav Bajtoš - the current maintainer, sponsored by StrongLoop.

Big thanks to the many contributors to the project, see AUTHORS.