Changes to process module in one file don't show up in another

The process module in NodeJS doesn't seem to be global. In other words, the changes I make to it in one module don't show up in other modules.

I wrote a small piece of code to confirm demo my findings. Here is the code:

server.js

import app from "./app.js";
 
process.on("uncaughtException", (err) => {
  console.log("UNCAUGHT EXCEPTION: shutting down ...");
  console.log(err.name, err.message);
  process.exit(1);
});
 
// Server
const port = process.env.PORT || 3000;
const server = app.listen(port, () => {
  console.log(`App running on port ${port}...`);
});

app.js

import express from 'express'
 
const app = express();
 
console.log(y)    
 
export default app;

When I run the code with node server.js, I get a following error:

file:///tmp/node-test/app.js:5
console.log(y);
            ^
 
ReferenceError: y is not defined
    at file:///tmp/node-test/app.js:5:13
    at ModuleJob.run (node:internal/modules/esm/module_job:193:25)

However when I move a line console.log(y) into server.js I get an error that looks like this:

UNCAUGHT EXCEPTION: shutting down ...
ReferenceError y is not defined

Which is what I expected to see when console.log(y) was in app.js.

It looks like changes I made in server.js with respect to process don't show up in app.js. I tried to find an answer online, but came up empty. My search foo wasn't good enough.

Does anyone know what's going on and how do I fix it? Because of coding standards I cannot use require, I must use import

When you start your program with this command:

node server.js

Then what happens is that server.js got compiled (so if it had a syntax error even at the end of the program by adding something obviously impossible to parse, like closing parens without opening ones like )))) then it would end at this point with error Uncaught SyntaxError: Unexpected token ')' or something like this. But syntax is fine. Who knows, a global variable y could be defined at the time it is actually executed.

Then, there is an import:

import app from "./app.js";

So now the app.js gets compiled. It is syntactically ok so it is executed. Only after it is executed then the rest of server.js could continue being executed. But app.js references a reference to y that is not defined, so the exception is thrown and the entire process exits.

This is the end of executing anything. So nothing below the import in file server.js gets executed (you can confirm it by adding console logs below the import), including the process.on() command.

Note:

For using ES modules (with import) the order of execution is:

1. you run node foo
2. foo.js has `import {} from './bar' (can be verified statically at compile time)
3. before even the first like of foo gets executed, the bar module is compiled and executed
4. only then the foo module gets executed

For using CommonJS (with require) the order of execution is:

1. you run node foo
2. foo.js gets compiled
3. if the compilation was ok foo starts getting executed
4. there is require('./bar')
5. so now bar starts getting executed
6. when bar finishes then foo continues getting executed (commands after require)

You can verify it on simple examples:

ES Modules:

foo.js:

console.log('foo 1');
import {} from './bar.js';
console.log('foo 2');

bar.js:

console.log('bar 1');
export const x = {};
console.log('bar 2');

CommonJS:

foo.js:

console.log('foo 1');
require('./bar.js');
console.log('foo 2');

bar.js:

console.log('bar 1');
module.exports = {};
console.log('bar 2');

Running node foo.js for ES Modules prints:

bar 1
bar 2
foo 1
foo 2

Running node foo.js for CommonJS prints:

foo 1
bar 1
bar 2
foo 2

You can play with it and add different kinds of compile time and run time errors and whaat is printed last.