.eleventy.js

module.exports = function(eleventyConfig) {
  eleventyConfig.addFilter('debug', function(...args) {
    debugger;
  });
}

package.json

{
  "name": "my project",
  "scripts": {
    "debug:eleventy": "node --inspect-brk ./node_modules/.bin/eleventy",
  }
}

page.njk

{{ data | debug }}

Running node with node --inspect-brk will put it into debug mode. If you have a page that uses the debug filter, then eleventy will halt execution when it processes that page. Whatever data you pass to that debug filter will show up when the debugger hits the debug statement on line 3 of .eleventy.js.

This post is a bit old but it explains the process: https://medium.com/@paul_irish/debugging-node-js-nightlies-with-chrome-devtools-7c4a1b95ae27

If you haven't used the node debugger before I'd suggest first creating a plain index.js file, and writing a few statements and adding a debugger statement in there to halt execution. See if you can get that working with node --inspect or node --inspect-brk. Once that seems to do what you expect, then try with eleventy. hope that helps!

oh maybe read this too: https://itnext.io/the-absolute-easiest-way-to-debug-node-js-with-vscode-2e02ef5b1bad
I forgot that I have the extension they mention in that post installed so my VS Code automatically connects the debugger if I start node with the --inspect-brk flag using VS Code's integrated terminal.

Thank you