Slow performance toLocaleString() Node.js update from 12.18.3 to 18.13.0

Question

We have recently updated node.js from version 12.18.3 to version 18.13.0. We run node.js on windows.

We experience a problem with the built-in function toLocaleString() and the problem is that it takes about 10x the time that it used to take earlier. The problem only occurs when we start our API as a service. If it's started in the command prompt and we shoot requests against it, the function's performance is as fast as it where earlier.

We have a function called strDateArr(fd, td) where we can send in a fromdate and todate and the function returns an array with string formatted dates. Example,

let dates = strDateArr("2013-01-31", "2022-12-31")

returns,

["2013-01-31", ..., "2022-12-31"]

The usage of toLocaleString() in our function looks like this.

let relDateStr = relDate.toLocaleString("sv-SE", {
  month: "2-digit",
  year: "numeric",
  day: "2-digit",
});

This might be a cache problem and I really appreciate any help. Thanks in advance.

Answer 1

You should use Intl.DateTimeFormat with its format() method when you want to format large numbers of dates. It usually is faster:

let formatter = new Intl.DateTimeFormat('sv-SE', {
  month: "2-digit",
  year: "numeric",
  day: "2-digit"
});

let relDateStr = formatter.format(relDate);

You can run the snippet below for a quick and dirty benchmark:

let dates = [];
let options = {
  month: "2-digit",
  year: "numeric",
  day: "2-digit"
};

for (let i = 0; i < 20 * 365; i++)
  dates.push(new Date(Date.now() - i * 24 * 60 * 60 * 1000));

let t0 = performance.now();

dates.map(date => date.toLocaleString("sv-SE", options));

let t1 = performance.now();
let formatter = new Intl.DateTimeFormat('sv-SE', options);

dates.map(date => formatter.format(date));

const t2 = performance.now();
console.log(`toLocaleString() took ${t1 - t0} milliseconds.`);
console.log(`Intl.DateTimeFormat() took ${t2 - t1} milliseconds.`);