Node.js - changing the value of an exported integer

Question

I have a file A.js where I have a module-level variable activeCount. I used module.exports to export it. I have a test file testA.js where I check the value of activeCount.

However it seems that the changes I make in A.js to activeCount aren't seen by testA.js. I think this is probably because when I change activeCount, it causes module.exports.activeCount and activeCount to point at different objects. Am I analyzing this correctly, and if so how do I change the value of activeCount without creating a new object?

A.js

var activeCount = 0;
...

function reset() {
    activeCount = 0;
}

function A() {
    ...
}
module.exports = A;
module.exports.activeCount = activeCount;
module.exports.reset = reset;

A.prototype.addFunction(...) {
    ...
    activeCount++;
    ...
}

testA.js

var A = require('A');

test('test1', function (assert) {
    var a = new A();
    a.addFunction(...);
    console.log(A.activeCount); // prints 0 instead of 1
});

test('test2', function (assert) {
    A.reset();
    var a = new A();
    a.addFunction(...);
    console.log(A.activeCount); // also prints 0 instead of 1
});

Answer 1

In Javascript, when you assign an integer (or any primitive - e.g. non-object), it is copied into the new variable. Thus a change to the new variable does not influence the original variable. Javascript does not have a true reference type for integers.

The usual work-around is to export an object (which is assigned by pointer) and then put the integer as a property on that object. Then, changing the property on the object will be seen by everyone who has a reference to that object.

As it turns out, you are already exporting activeCount as a property of an object (since it is a property of module.exports in the A.js module. So, you just need to change the "A" module to use it from there rather than use it's local copy. There are a couple different ways to do that. Here's one:

// A.js
function reset() {
    // change the exported variable
    module.exports.activeCount = 0;
}

function A() {
    ...
}
module.exports = A;
module.exports.activeCount = 0;
module.exports.reset = reset;

A.prototype.addFunction(...) {
    ...
    // change the exported variable
    module.exports.activeCount++;
    ...
}

Now, your testA.js module will work as expected.

---

Note, since you are doing module.exports = A; and functions are objects, you could also reference activeCount as a property of A while inside the A module and it would also solve the problem and give the desired result:

// A.js
function reset() {
    // change the exported variable
    A.activeCount = 0;
}

function A() {
    ...
}

// add properties to our constructor function so those properties
// are also exported
A.activeCount = 0;
A.reset = reset;
module.exports = A;

A.prototype.addFunction(...) {
    ...
    // change the exported variable
    module.exports.activeCount++;
    ...
}