How to get dynamic value of a variable declared in another file in NodeJS

Question

In app.js file:

const tempStore = require("./tempStore.js");

setInterval(() => {
   tempStore.setTemp(1);
   console.log(tempStore.temp); // I expect this will log 1 then 2 then 3 so on...
}, 1000);

In tempStore.js file:

let temp = 0;

const setTemp = num => {
   temp += num;
}

module.exports = {
   temp: temp,
   setTemp: setTemp
}

I expect this line console.log(tempStore.temp); will give me a sequence of increasing number:

1
2
3
4
...
...

But it gives me this:

0
0
0
0
..
..

In other words always 0.

I can find another way to get what I expect by modifying this code:

In app.js file:

const number = tempStore.setTemp(1); // store returned value in a constant
console.log(number); // show it

In tempStore.js file:

temp += num;
return temp; // return the result

But I prefer to get the number directly from temp, why this can't be done?

As far as I remember I can do this while coding in front-end development between .js file. But why I can't do this in NodeJS, what's wrong?

because the export is only the value at the time it is read. export.temp is not the same as temp You should be using a getter. — epascarello
– epascarello, Commented Oct 18, 2019 at 13:44
@epascarello Could you provide some link to reference about that? — Diamond
– Diamond, Commented Oct 18, 2019 at 13:45

Titus · Accepted Answer · 2019-10-18 13:51:01Z

3

Try something like this:

const store = {
   temp: temp,
   setTemp: setTemp
}

function setTemp(num) {
   store.temp += num;
}

module.exports = store;

The problem with your current approach is that the exported object's temp property is set to the value of temp when the object is created, and since inside the setTemp function the only change you make is to reassign temp, the exported object will not be mutated.

Is the same as this:

let temp = 0;

const setTemp = num => {
   temp += num;
}

const store = {
   temp: temp,
   setTemp: setTemp
}

console.log("default", store.temp);

setTemp(10);

console.log("after change", store.temp);

edited Oct 18, 2019 at 13:51

answered Oct 18, 2019 at 13:45

Titus

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