Do not use magic numbers when manipulating strings

Written byPhuoc Nguyen

Tags

JavaScript

Created

09 Mar, 2021

It's quite often that we pass the index parameter when using string manipulation functions such as `slice`, `splice`, `substr` or `substring`.

Let's take a look at a simple example which converts a given date object to YYYY-MM-DD format.

Given a `date` object, we can get the string representing the date in the ISO format:

date.toISOString();
// '2021-03-09T01:42:46.002Z'

The output is constructed by different parts which starts with the full date, `2021-03-09` as we see in the example above. So, it's easy to get the full date by extracting it from the ISO format:

const format = (date) => date.toISOString().slice(0, 10);

`10` is called a magic number, because when the other people look at the code, they don't have idea where the number comes from.

Rather than using a magic number, it's recommended to explain what it is and how it's calculated:

// The better version
const FULL_DATE_LENGTH = 'YYYY-MM-DD'.length;

const format = (date) => date.toISOString().slice(0, FULL_DATE_LENGTH);

#See also

Keep the calculation of a magic number

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Phước Nguyễn

Do not use magic numbers when manipulating strings

#See also

Questions? 🙋

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