Keep the calculation of a magic number

Let's say that we want to calculate the number of difference days between two given dates. By simply using the `-` operator, it's easy for us to calculate the differences between two date objects, `date` and `otherDate` for example:
const diff = Math.abs(date - otherDate);
The function returns the number of milliseconds. In order to get the number of days, we have to divide it by `86400000`:
Math.ceil(diff / 86400000);
We know that `86400000` is the total number of milliseconds in a day. The code is still working fine until there's another people on the team looking at it, and it takes them time to realize what the number `86400000` is.
To make the code more readable and easy to maintain, we should replace the magic number with its calculation:
// Better version
const diffDays = (date, otherDate) => Math.ceil((Math.abs(date - otherDate) / 1000) * 60 * 60 * 24);
If the number is used across different places, for example when we create a library for manipulating dates, then it's recommended to declare the magic number as a constant:
// The best version
// constants.js
const MILLISECOND_DAY = 1000 * 60 * 60 * 24;
Then reuse it later:
import { MILLISECOND_DAY } from './constants';
const diffDays = (date, otherDate) => Math.ceil(Math.abs(date - otherDate) / MILLISECOND_DAY);

See also