Do not use magic numbers when manipulating strings
It's quite often that we pass the index parameter when using string manipulation functions such as
date object, we can get the string representing the date in the ISO format:
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:
const FULL_DATE_LENGTH = 'YYYY-MM-DD'.length;
const format = (date) => date.toISOString().slice(0, FULL_DATE_LENGTH);