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1.12.1-a • Public • Published

niim - Node-Inspect IMproved

npm install --global niim


This project is a fork of the node-inspect project. The goal is simple: to scratch my own itches with respect to debugging command-line apps on NodeJS, especially logged in via ssh.

Specifically, my work on DCP, a platform for doing massively-parallel computation in JavaScript, has been frustrated by the lack of a text-ui debugger which works with our library that unlocks Ethereum keystores. See Distributed.Computer for more information if you're curious about that.

Major Functional Differences From Node-Inspect

  • Can debug processes that require input on stdin, including REPLs
  • Avoid startup pause can be disabled by default via config on a per-target basis
  • Use randomized inspect port by default
  • REPL history is maintained; inner REPL history is per program name


  • Most plans are laid out in the GitHub issue tracker. (Feel free to help out!)
  • I will be tracking changes to node-inpsect; further divergence from node-inspect will be primarily centered on usability
  • UX issues will help differentiate stdout from stderr, debugger text from debuggee text via colour, improve error messaging, reduce stack trace clutter, etc.
  • The config file should be fairly stable, but will continue to grow settings, particularly those for helping to put niim into certain modes (like "break on uncaught") for specific programs.
  • Version numbers no longer track node-inspect.
  • Bugfixes related to core functionality will be implemented in feature branches that should be easy for the main project to merge.

Release Status

This fork is barely past the "proof of concept" stage. Please be aware that it is barely tested on my machines, let alone yours. I'm running Node 10.20 on Linux x86_64 at work and Node 14 on macOs Catalina at home. Please let me know if you're using niim and find it useful - and feel free to suggest ideas!

Launching Niim

niim [options] ‹filename to debug›

Option Behaviour
--config-file= Specify an additional config file (overlays etc/config)
--port Specify the port to use for the node-inspect protocol. Default: auto
--host Specify the hostname to use for the node-inspect protocol. Default: localhost

Debugging with niim

All of the commands from node-inspect work as usual. If you are on an older version of NodeJS, you might find that they work better than usual. :)

New Command Behaviour
send(string) Send the string to the attached process' stdin
sendFile(filename) Send the named file to the attached process' stdin
pipe(fd or command) Send the data on the pipeline to the attach process' stdin
ctty Suspend the REPL and enter interactive termimal mode

There is also a FAQ in this directory which goes into more detail.


Interactive Terminal Mode

This feature is the raison d'être for this fork, as our team frequently finds itself needing to enter passphrases during our debugging sessions.

In this mode, the REPL is suspended and the debugger's stdin is fed to the attached process. If the attached process is in raw mode, the debugger's terminal will also be set in raw mode. During interactive terminal mode, the debugger will also not print symbols in front of the attached process' stdout.

The niim preloader communicates with niim using its own protocol, over the attached process' stdout. All messages are of the form NUL{json}NUL. If the attached process writes a NUL, it is escaped with a second NUL.

To facilitate this mode, the niim preloader monkey-patches and otherwise tries to virtualize APIs on process.stdin and process.stdout. Setting DEBUG_NIIM and/or DEBUG_NIIM_PRELOAD can yield insight into the under-the-hood behaviour if strange things are happening for you.

It is very important that a process under debugging only use the stdout Stream interface for writing to stdout if the data written can contain NUL characters.

niim module

The internal module require("niim") is supplied to the attached process via the niim preloader. This library allows for niim-aware debug targets to interoperate with niim directly.

Module export Behaviour
itm(boolean) true - enter interactive terminal mode.
false - exit interactive terminal mode.


niim ships with a niim.config master configuration in the etc/ directory of the package to describe all of the configuration options and their defaults. The config files are read in the following order; the last file read that sets a given property has precedence:

  • etc/niim.config
  • etc/your-program-name.config
  • ~/.niim/config
  • ~/.niim/your-program-name.config
  • filename passed with --config=
Environment Variable Behaviour
NIIM_CONFIG_FILE Specify an additional config file (overlays etc/config)
NIIM_DEFAULT_PORT Specify the default port to use for the node-inspect protocol.
Default: auto
NIIM_REPL_HISTORY Alternative history filename
NIIM_REPL_SIZE Override history size

Enabling Autostart

If your work flow does not involve setting breakpoints the moment niim launches, you might like to enable autostart; this feature skips the first niim› prompt and starts running the attached process right away.

To enable autostart globally, set niim.autostart=true in ~/.niim/config. To enable it only when debugging a program named XYZ, set niim.autostart=true in ~/.niim/XYZ.config.

Other Debuggers

The fork root, node-inspect, is maintained by the NodeJS team. This is the debugger you launch with node debug filename.js. The latest version of node-inspect is available at https://github.com/nodejs/node-inspect. The principal author is Jan Krems, jan.krems@gmail.com.

The V8 team maintain an excellent browser-based GUI debugger; it is available at https://github.com/node-inspector/node-inspector.



npm i niim

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