Debugger

Added in: v0.9.12

Node.js includes a command-line debugging utility. The Node.js debugger client is not a full-featured debugger, but simple stepping and inspection are possible.

To use it, start Node.js with the inspect argument followed by the path to the script to debug.

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The debugger automatically breaks on the first executable line. To instead run until the first breakpoint (specified by a debugger statement), set the NODE_INSPECT_RESUME_ON_START environment variable to 1.

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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

• cont, c: Continue execution
• next, n: Step next
• step, s: Step in
• out, o: Step out
• pause: Pause running code (like pause button in Developer Tools)

Breakpoints

• setBreakpoint(), sb(): Set breakpoint on current line
• setBreakpoint(line), sb(line): Set breakpoint on specific line
• setBreakpoint('fn()'), sb(...): Set breakpoint on a first statement in function's body
• setBreakpoint('script.js', 1), sb(...): Set breakpoint on first line of script.js
• setBreakpoint('script.js', 1, 'num < 4'), sb(...): Set conditional breakpoint on first line of script.js that only breaks when num < 4 evaluates to true
• clearBreakpoint('script.js', 1), cb(...): Clear breakpoint in script.js on line 1

It is also possible to set a breakpoint in a file (module) that is not loaded yet:

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It is also possible to set a conditional breakpoint that only breaks when a given expression evaluates to true:

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Information

• backtrace, bt: Print backtrace of current execution frame
• list(5): List scripts source code with 5 line context (5 lines before and after)
• watch(expr): Add expression to watch list
• unwatch(expr): Remove expression from watch list
• watchers: List all watchers and their values (automatically listed on each breakpoint)
• repl: Open debugger's repl for evaluation in debugging script's context
• exec expr, p expr: Execute an expression in debugging script's context and print its value

Execution control

• run: Run script (automatically runs on debugger's start)
• restart: Restart script
• kill: Kill script

Various

• scripts: List all loaded scripts
• version: Display V8's version

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 DevTools 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.

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(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.)

If the Chrome browser is older than 66.0.3345.0, use inspector.html instead of js_app.html in the above URL.

Chrome DevTools doesn't support debugging worker threads yet. ndb can be used to debug them.