Debugging Guide

This guide will help you get started debugging your Node.js apps and scripts.

Enable Inspector

NOTE: The --inspect option and Inspector Protocol are experimental and may change.

When started with the --inspect switch, a Node.js process listens via WebSockets for diagnostic commands as defined by the Inspector Protocol, by default at host and port 127.0.0.1:9229. Each process is also assigned a unique UUID (e.g. 0f2c936f-b1cd-4ac9-aab3-f63b0f33d55e).

Inspector clients must know and specify host address, port, and UUID to connect to the WebSocket interface. The full URL is ws://127.0.0.1:9229/0f2c936f-b1cd-4ac9-aab3-f63b0f33d55e, of course dependent on actual host and port and with the correct UUID for the instance.

Inspector also includes an HTTP endpoint to serve metadata about the debuggee, including its WebSocket URL, UUID, and Chrome DevTools URL. Get this metadata by sending an HTTP request to http://[host:port]/json/list. This returns a JSON object like the following; use the webSocketDebuggerUrl property as the URL to connect directly to Inspector.

{
  "description": "node.js instance",
  "devtoolsFrontendUrl": "chrome-devtools://devtools/bundled/inspector.html?experiments=true&v8only=true&ws=127.0.0.1:9229/0f2c936f-b1cd-4ac9-aab3-f63b0f33d55e",
  "faviconUrl": "https://nodejs.org/static/favicon.ico",
  "id": "0f2c936f-b1cd-4ac9-aab3-f63b0f33d55e",
  "title": "node",
  "type": "node",
  "url": "file://",
  "webSocketDebuggerUrl": "ws://127.0.0.1:9229/0f2c936f-b1cd-4ac9-aab3-f63b0f33d55e"
}

A Node.js process started without --inspect can also be instructed to start listening for debugging messages by signaling it with SIGUSR1 (on Linux and OS X). As of Node 7 this activates the legacy Debugger API; in Node 8 and later it will activate the Inspector API.


Inspector Clients

Several commercial and open source tools can connect to Node's Inspector. Basic info on these follows:

node-inspect

  • CLI Debugger supported by the Node.js Foundation which uses the Inspector Protocol.
  • A version is bundled with Node and can be used with node inspect myscript.js.
  • The latest version can also be installed independently (e.g. npm install -g node-inspect) and used with node-inspect myscript.js.

Chrome DevTools 55+

VS Code 1.10+

  • In the Debug panel, click the settings icon to open .vscode/launch.json. Select "Node.js" for initial setup.

JetBrains WebStorm 2017.1+ and other JetBrains IDEs

  • Create a new Node.js debug configuration and hit Debug. --inspect will be used by default for Node.js 7+. To disable uncheck js.debugger.node.use.inspect in the IDE Registry.

chrome-remote-interface

  • Library to ease connections to Inspector Protocol endpoints.

Command-line options

The following table lists the impact of various runtime flags on debugging:

FlagMeaning
--inspect
  • Enable inspector agent
  • Listen on default address and port (127.0.0.1:9229)
--inspect=[host:port]
  • Enable inspector agent
  • Bind to address or hostname host (default: 127.0.0.1)
  • Listen on port port (default: 9229)
--inspect-brk
  • Enable inspector agent
  • Listen on default address and port (127.0.0.1:9229)
  • Break before user code starts
--inspect-brk=[host:port]
  • Enable inspector agent
  • Bind to address or hostname host (default: 127.0.0.1)
  • Listen on port port (default: 9229)
  • Break before user code starts
node inspect script.js
  • Spawn child process to run user's script under --inspect flag; and use main process to run CLI debugger.

Legacy Debugger

The legacy debugger has been deprecated as of Node 7.7.0. Please use --inspect and Inspector instead.

When started with the --debug or --debug-brk switches in version 7 and earlier, Node.js listens for debugging commands defined by the discontinued V8 Debugging Protocol on a TCP port, by default 5858. Any debugger client which speaks this protocol can connect to and debug the running process; a couple popular ones are listed below.

The V8 Debugging Protocol is no longer maintained or documented.

Built-in Debugger

Start node debug script_name.js to start your script under Node's builtin command-line debugger. Your script starts in another Node process started with the --debug-brk option, and the initial Node process runs the _debugger.js script and connects to your target.

node-inspector

Debug your Node.js app with Chrome DevTools by using an intermediary process which translates the Inspector Protocol used in Chromium to the V8 Debugger protocol used in Node.js.

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