I declared a global variable a in lib.js and exported 2 functions init_a and return_a from the lib.js.

Calling init_a will initialise the undefined a into value (I used date time for the example).

Calling return_a will first check if a is defined and return it immediately. Otherwise, it will call init_a before returning it.

I am having a little confusion now because I couldn’t understand why a is giving me different value depending on whethere init_a is called explicitly or not. Following are the source codes:

// lib.js
let a;

function init_a() {
  const init_time = new Date().getTime();
  console.log(`init_a ${init_time}`);
  a = init_time;
}

function return_a() {
  console.log(`return_a`);
  if (!a) {
    console.log(`No a found, creating a new a`);
    init_a();
  }

  console.log(`return_a return`);
  return a;
}

module.exports = {
  init_a,
  return_a,
};

// main.js
const { init_a, return_a } = require("./lib");

function main() {
  console.log("[DEBUG] main");
  // init_a();

  a = return_a();
  console.log(a);
}

module.exports = main;

// main2.js
const { init_a, return_a } = require("./lib");

function main() {
  console.log("[DEBUG] main2");
  init_a();

  a = return_a();
  console.log(a);
}

module.exports = main;

// index.js
const main = require("./main");
const main2 = require("./main2");

async function __main__() {
  main();
  await sleep(1000);
  main2();
}

function sleep(ms) {
  return new Promise((resolve) => setTimeout(resolve, ms));
}

(async () => {
  await __main__();
})();

Output of node index.js

[DEBUG] main
return_a
No a found, creating a new a
init_a 1721288726594
return_a return
1721288726594
[DEBUG] main2
init_a 1721288727596
return_a
return_a return
1721288727596

Here we noticed the value of a is different from the context of main and main2

Output of node index.js after commenting init_a in main2.js

[DEBUG] main
return_a
No a found, creating a new a
init_a 1721288786523
return_a return
1721288786523
[DEBUG] main2
return_a
return_a return
1721288786523

Here we noticed the value of a is the same in both the context of main and main2

—
I couldn’t wrap my head around the behavior. Can someone please enlighten me :).

I have created a repo that contains all of the files, you can simply clone and run with node index.js in your terminal!
https://github.com/tanshinjie/stackoverflow-qn-on-js-scope.git

Thank you!

I have read about lexical scoping in JS but I am not exactly sure if that is what happening in this case.