setInverval lagging behind, and Heroku - node.js

Question

This is the code that I deployed on Heroku. It's function is to update a certain key that i'm displaying every ten seconds.
const express = require('express');
const timestamp = require('time-stamp');
var app = express();
var port = 3000;
var server = app.listen(process.env.PORT || port, '0.0.0.0', function() {
console.log(`Listening at port: ${port}`);
});
app.get("/", encr)
code = 5
function encr(req, res){
console.log(`Code accessed, code = ${code}`)
res.send({code})
}
setInterval(async function() {
code = Math.floor((Math.random() * 100) + 1);
console.log(`Key updated. Time: ${timestamp.utc('DD/mm:ss:ms')}`)
}, 10 * 1000);
This setInterval function needs to run every 10 seconds, but ends up running every 10.01 seconds, which after a bit begins to add up.
Logs:
2021-02-08T20:15:45.414054+00:00 app[web.1]: Key updated. Time: 08/15:45:413
2021-02-08T20:15:55.423763+00:00 app[web.1]: Key updated. Time: 08/15:55:423
[...]
2021-02-08T20:18:25.543898+00:00 app[web.1]: Key updated. Time: 08/18:25:543
2021-02-08T20:18:35.551720+00:00 app[web.1]: Key updated. Time: 08/18:35:551
2021-02-08T20:18:45.559856+00:00 app[web.1]: Key updated. Time: 08/18:45:559
How can I fix this so it's exactly 10 seconds?

Answer 1

Timeouts/intervals aren't entirely accurate at the millisecond timescale. Instead of trying to reassign code every precise interval, consider instead saving the timestamp whenever code gets updated, and when it gets accessed again, check whether the timestamp has expired - if so, reassign it:
const makeCode = () => {
code = Math.floor((Math.random() * 100) + 1);
lastAssignTimestamp = performance.now();
};
let lastAssignTimestamp;
let code;
makeCode();
function encr(req, res){
if (performance.now() - lastAssignTimestamp > 10_000) {
// then the code has expired...
makeCode();
}
console.log(`Code accessed, code = ${code}`)
res.send({code})
}

Answer 2

I've edited my code to also print the last update's timestamp, that way I can just check directly in my other code without time keeping shenanigans
const express = require('express');
var app = express();
var port = 3000;
var server = app.listen(process.env.PORT || port, '0.0.0.0', function() {
console.log(`Listening at port: ${port}`);
});
app.get("/", encr)
code = 5
last_time = 0
const makeCode = () => {
code = Math.floor((Math.random() * 100) + 1);
lastAssignTimestamp = +new Date()
console.log(`New Code: ${code}`)
};
let lastAssignTimestamp;
makeCode();
setInterval(function(){
let x = +new Date()
if (x - lastAssignTimestamp > 1000 * 60){
// then the code has expired... every minute
makeCode();
}
last_time = lastAssignTimestamp
console.log(`Time left till next update: ${Math.round(((1000 * 60) - (x - lastAssignTimestamp))/1000)}`)
}, 1000);
function encr(req, res){
res.send(
{code, last_time}
)
}