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 versionconst 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.jsconst MILLISECOND_DAY = 1000 * 60 * 60 * 24;export { MILLISECOND_DAY };

Then reuse it later:

import { MILLISECOND_DAY } from './constants';const diffDays = (date, otherDate) => Math.ceil(Math.abs(date - otherDate) / MILLISECOND_DAY);