Why doesn't a JavaScript variable update across files as expected?

Question

Why doesn't a JavaScript variable update across files as expected?

I am facing an issue with two files in my project:

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 cannot figure out why the value of list is not updating in test.js. The workaround I found is to use list.push("item"); instead of list = ["item"];. This behavior is puzzling to me.

javascript arrays memory

Answer 1

Answer №1

When you bring in an array (or object, ...) from another module, you are essentially establishing a connection to that array. By using list = ["item"]; in your index.js file, you are reassigning the value of list, while .push modifies the original list in index.js as well as the reference to it in test.js. This reassignment severs the link between index.js and test.js. As a result, index.js continues to point to the original empty array, whereas test.js now points to the updated array.

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);

Answer 2

When you bring in an array (or object, ...) from another module, you are essentially establishing a connection to that array. By using list = ["item"]; in your index.js file, you are reassigning the value of list, while .push modifies the original list in index.js as well as the reference to it in test.js. This reassignment severs the link between index.js and test.js. As a result, index.js continues to point to the original empty array, whereas test.js now points to the updated array.

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);

Answer 3

Answer №2

The code you've written assigns a brand new array to the variable list. This means that the exported value was originally pointing to the original array. When you update the local variable within the "index" module, it does not impact the variable within the "test" module.

This issue isn't related to importing and exporting between modules. The same scenario can be observed in this example:

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

Changing the reference of variable a to a new array doesn't change the value of variable b. This behavior is fundamental to how JavaScript operates.

Answer 4

The code you've written assigns a brand new array to the variable list. This means that the exported value was originally pointing to the original array. When you update the local variable within the "index" module, it does not impact the variable within the "test" module.

This issue isn't related to importing and exporting between modules. The same scenario can be observed in this example:

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

Changing the reference of variable a to a new array doesn't change the value of variable b. This behavior is fundamental to how JavaScript operates.

Why doesn't a JavaScript variable update across files as expected?

Answer №1

Answer №2

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