Javascript variable export unexpected behavior - why doesn't it update across files?

I have two files here:

index.js

let list = [];

function add() {
    //list.push("item");
    list = ["item"];
    console.log("B. list length " + list.length);
}

module.exports = {
    add,
    list
}

test.js

let { add, list } = require('./index');

console.log("A. list length " + list.length);
add();
console.log("C. list length " + list.length);

Current output:

A. list length 0
B. list length 1
C. list length 0

Expected output:

A. list length 0
B. list length 1
C. list length 1

I don't understand why list not being updated in test.js.
The one fix I found was to use list.push("item"); instead of list = ["item"];.
I have no idea why this is happening.

When importing an array (or object, ...) from another module, you're actually creating a reference to that array. Using list = ["item"]; in your index.js file you reassign list whilst .push modifies the list in index.js and also the list reference you import in test.js. Reassignment breaks the reference between index.js and test.js. index.js still refers to the original empty array, while test.js now has a reference to the new array

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

let list = [];

function add() {
  // list.push("item");
  list = ["item"];
  console.log("B. list length " + list.length);
}

const importedList = list; // mocking require("./index");

console.log("A. importedList length " + importedList.length);
add();
console.log("C. importedList length " + importedList.length);

<!-- end snippet -->

Your code assigns a completely new array to the value of list. However, the exported value was a reference to the original array. Updating the local variable in the "index" module does not change the value of the variable in the "test" module, in other words.

The problem really has nothing to do with import and export across module boundaries. It is the same situation here:

let a = [1, 2, 3];
let b = a;
a = [4, 5, 6];
console.log(b); // [1, 2, 3]

Making the variable a reference a new array does not affect the value of variable b. That's a basic feature of how JavaScript works.

To achieve the expected behavior, where the list length is updated correctly in test.js, you should use the push method or another method that modifies the existing array, instead of assigning a new array. Here's how you can do it:

// test.js
const list = [];

export function addItemToList(item) {
    list.push(item); // Add the item to the existing array
}

export function getListLength() {
    return list.length;
}