NPM npm_package_main variable is always empty

Question

Problem

NPM $npm_package_main variable is always empty.

When I set the package.json file with "main": "index.js"
Set the "start" property from scripts to "start": "node $npm_package_main"
Then run npm start

Problem: the CLI executes the Node REPL mode, ignoring the "main" variable from package.json.

Expected behavior: execute the command as node index.js.

Environment

Linux Ubuntu 20.04.1
npm -v = 7.3.0
node -v = v15.5.0
npm run env | grep npm_package_name = npm_package_name=app
npm run env | grep npm_package_main = EMPTY

How to reproduce

Create an "app" directory and enter the new directory
Create an "index.js" file with the following content console.log('HELLO');
Run npm init and hit ENTER for all questions
Edit the package.json file and add the following line to the "scripts" property: "start": "node $npm_package_main",
now your package.json must look like this

    {
      "name": "app",
      "version": "1.0.0",
      "description": "",
      "main": "index.js",
      "devDependencies": {},
      "scripts": {
        "start": "node $npm_package_main",
        "test": "echo \"Error: no test specified\" && exit 1"
      },
      "author": "",
      "license": "ISC"
    }

Run "npm start"
File "index.js" is not executed and Node enters the REPL mode.

Attempts

Set "start" and running "npm start" for:

"echo $npm_package_main" prints nothing
"echo $npm_package_name" prints "app"
"echo $npm_package_version" prints "1.0.0"

References

NPM package.json variables: https://docs.npmjs.com/cli/v7/using-npm/scripts
NPM Github issue: https://github.com/npm/cli/issues/2585

user_3010938 · Answer

The official answer from NPM: use "node .". There's no official reason for this behavior, until this post.
According to the documentation, the "main" property contains the entrypoint for your app when is used as a module in other projects: https://docs.npmjs.com/cli/v7/configuring-npm/package-json#main

Solution

Use the "config" property instead:

{
  "name": "app",
  "version": "1.0.0",
  "description": "",
  "devDependencies": {},
  "config": {
    "main": "index.js"
  },    
  "scripts": {
    "start": "node $npm_package_config_main",
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC"
}

Run "npm start"
It works