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;

export { MILLISECOND_DAY };

Then reuse it later:

`import { MILLISECOND_DAY } from './constants';`

const diffDays = (date, otherDate) => (

Math.ceil(Math.abs(date - otherDate) / MILLISECOND_DAY)

);

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