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Better Node Inspect: A Better Node Command Line Debugger

Let's make node inspect better on the command line! I'm used to amazing CLI debugging tools like ipython, iex, and pry. I want to bring some of that magic to node.

Goals

This isn't going to all be possible, but here's my wishlist:

  • Returning a object in the REPL prints the whole object, rather than a truncated version of the object with ...
  • Easily copy content (JSON representation of a object, large strings, etc) into the clipboard
  • Optionally support printing long output to a pager
  • Ability to execute typescript in the repl, ignoring any typing errors (transpilation only)
  • Colorized output.
  • Ability to use await in a debugger session (SyntaxError: await can not be used when evaluating code while paused in the debugger)
  • Automatically pause on an exception and keep all state
  • Ability to reload code, even if it's already been loaded. With ESM and a custom loader (possibly built on top of esbuild's ESM loader), this should be possible.
  • Ability to define a function dynamically with function name() {}
  • Ability to paste in a multiline command
  • Tab completion for locally defined variables
  • Ability to print without leading > in debug mode
  • Show stack trace in debugger when it's seems to be doing nothing
  • I don't want my imports rewritten from log into loglevel_1
  • Shortcut to open my editor at the file:line:column
  • Ability to define custom debugger shortcuts (i.e. c vs cont)
  • _ does not work in a debugger repl, I want to get the last result using that shortcut
  • Drop into a repl immediately instead of a debug> session, or execute certain types of code within the inner repl automatically.
  • Custom backtrace filters using a middleware-like interface
  • _ex_ exception reference when an exception is raised
  • up and down stacktrace navigation
  • If there's an exception, but the original source is not present, jump to the source in github
  • File location follows next, up, etc. By default, this does not seem to happen.
  • Sourcemap-aware file and line locations. If you use tsx, or any other transpilation tool, the source and line + column numbers are based on the transpiled code, not the original source.
  • Avoid process.stdout breaking when certain content is passed to it. When the debugger is active, the event loop is paused, and the drain event is not processed causing process.stdout.write to stop working entirely.
  • Works with test runners like jest and vitest
  • Automatically start a debugger session when an exception is thrown in a test

What's done

  • up and down stacktrace navigation
  • frame(n) to jump to a specific stack frame
  • colorized output
  • pretty print objects using pp
  • list local variables with locals
  • list current file location with context using ls
  • get current file location using location
  • set default exception state via --pause-on-exception-state
  • don't pause at the beginning of execution via --inspect-resume-on-start
  • set node inspect specific CLI arguments using NODE_INSPECT_OPTIONS env var (export NODE_INSPECT_OPTIONS="--inspect-resume-on-start true --pause-on-exception-state uncaught")

Upstream PRs

One of my biggest gripes with the nodejs ecosystem is how fragmented it is. My goal is to get all of the changes here upstreamed:

Usage

Install:

npm install better-node-inspect --save-dev

In order to use the improved inspector:

better-node-inspect the-file.js

To use the repl utilities make sure you import the module:

import "better-node-inspect"

Troubleshooting

If you are running into issues running the script, check to make sure NODE_OPTIONS doesn't contain any additional config. Sometimes VS Code can inject NODE_OPTIONS into the environment, which can cause issues. Here's the error I got when NODE_OPTIONS was set by VS Code.

node:internal/assert:14
    throw new ERR_INTERNAL_ASSERTION(message);
    ^

Error [ERR_INTERNAL_ASSERTION]: This is caused by either a bug in Node.js or incorrect usage of Node.js internals.
Please open an issue with this stack trace at https://github.com/nodejs/node/issues

    at new NodeError (node:internal/errors:405:5)
    at assert (node:internal/assert:14:11)
    at setupUserModules (node:internal/process/pre_execution:126:3)
    at prepareExecution (node:internal/process/pre_execution:118:5)
    at prepareMainThreadExecution (node:internal/process/pre_execution:42:3)
    at Object.<anonymous> (/Users/mike/Projects/javascript/better-node-inspect/main/inspect.js:10:1)
    at Module._compile (node:internal/modules/cjs/loader:1257:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1311:10)
    at Module.load (node:internal/modules/cjs/loader:1115:32)
    at Module._load (node:internal/modules/cjs/loader:962:12) {
  code: 'ERR_INTERNAL_ASSERTION'
}

Developing on node inspect

Where Things Are

It's always helpful to understand how things are wired together when you are hacking:

Inspiration

There's been a lot of attempts to improve the node REPL over the years: