How do node_modules packages read config files in the project root?

First search in path.dirname(process.mainModule.filename) then go up the directory tree ../, ../../, ../../../ and so on until you find the configuration file.

Here is the code I use stolen from rc (https://github.com/dominictarr/rc) package, it will read and json parse configuration from a file named .projectrc:

const fs = require('fs');
const path = require('path');

// Utils shamefully stolen from
// https://github.com/dominictarr/rc/blob/master/lib/utils.js

find(...args) {
  const rel = path.join.apply(null, [].slice.call(args));
  return findStartingWith(path.dirname(process.mainModule.filename), rel);
}

findStartingWith(start, rel) {
  const file = path.join(start, rel);
  try {
    fs.statSync(file);
    return file;
  } catch (err) {
    // They are equal for root dir
    if (path.dirname(start) !== start) {
      return findStartingWith(path.dirname(start), rel);
    }
  }
}

parse(content) {
  if (/^\s*{/.test(content)) {
    return JSON.parse(content);
  }
  return undefined;
}

file(...args) {
  const nonNullArgs = [].slice.call(args).filter(arg => arg != null);

  // path.join breaks if it's a not a string, so just skip this.
  for (let i = 0; i < nonNullArgs.length; i++) {
    if (typeof nonNullArgs[i] !== 'string') {
      return;
    }
  }

  const file = path.join.apply(null, nonNullArgs);
  try {
    return fs.readFileSync(file, 'utf-8');
  } catch (err) {
    return undefined;
  }
}

json(...args) {
  const content = file.apply(null, args);
  return content ? parse(content) : null;
}

// Find the rc file path
const rcPath = find('.projectrc');
// Or
// const rcPath = find('/.config', '.projectrc');

// Read the contents as json
const rcObject = json(rcPath);
console.log(rcObject);

You can also use the rc package as a dependency npm i rc, then in your code:

var configuration = require('rc')(appname, {
  // Default configuration goes here
  port: 2468
});

This will read config from a file named .${appname}rc.

They start from the directory the file lives in and recursively looks upwards in the file system tree until it finds the file it's looks for.

Something like this:

const FILE_NAME = 'target-file.json';

const fsp = require('fs').promises,
      path = require('path');

let find = async (dir=__dirname) => {
  let ls = await fsp.readdir(dir);
  if(ls.includes(FILE_NAME))
    return path.join(dir,FILE_NAME);
  else if(dir == '/')
    throw new Error(`Could not find ${FILE_NAME}`);
  else
    return find(path.resolve(dir,'..'));
}

Or if you were looking for a standard node "project root" you might want to recurse up and find a directory containing a directory names 'node_modules' like this:

const fsp = require('fs').promises,
      path = require('path');

let find = async (dir=__dirname) => {
  let ls = await fsp.readdir(dir);
  if(ls.includes('node_modules'))
    return dir;
  else if(dir == '/')
    throw new Error(`Could not find project root`);
  else
    return find(path.resolve(dir,'..'));
}

There are multiple ways to do it. I've created a test-package, and a demo project node-package-test to test it.

For ready reference providing the main code here:

project-main\node_modules\test-package\index.js :

const path = require('path');
const fs = require('fs');

const CONFIG_NAME = 'cfg.json';

function init(rootDir = null) {
  console.log(`test-package: process.cwd(): ${process.cwd()}`);
  console.log(`test-package: path.resolve('./'): ${path.resolve('./')}`);

  if (!rootDir) {
    //rootDir = path.resolve('./');
    // OR
    rootDir = process.cwd();
  }

  //const configPath = path.resolve('./', CONFIG_NAME);
  // OR
  const configPath = path.join(rootDir, CONFIG_NAME);


  if (fs.existsSync(configPath)) {
    console.log(`test-package: Reading config from: ${configPath}`);
    try {
      //const data = fs.readFileSync(configPath, 'utf8');
      //const config = JSON.parse(data);
      // OR
      const config = require(configPath);
      console.log(config);
    } catch (err) {
      console.error(err)
    }
  } else {

    console.log(`test-package: Couldn't find config file ${configPath}. Using default.`)
  }

  console.log('\n')
}

//init()
const features = {
  init: init,
  message: `Hello from test-package! `
}


module.exports = features;

project-main\main.js :

const utils = require('@onkarruikar/test-package')

utils.init();
// OR use
//utils.init('/path/to/rootdir');

console.log(`${utils.message}`);

Output:

E:\node-package-test-main>npm install

added 1 package, and audited 2 packages in 4s

found 0 vulnerabilities

E:\node-package-test-main>npm start

> start
> node .

test-package: process.cwd(): E:\node-package-test-main
test-package: path.resolve('./'): E:\node-package-test-main
test-package: Reading config from: E:\node-package-test-main\cfg.json
{ compilerOptions: { allowJs: true, checkJs: true, noEmit: true } }


Hello from test-package!