30-seconds-of-code/snippets/mask.md

mask

Replaces all but the last num of characters (first if num is negative, by default 4) with the provided mask character ('*' by default).

Use Array.slice() to grab the portion of the characters that need to be masked and use Array.replace() with a regex to replace every character with the mask character. Concatenate the masked characters with the remaining unmasked portion of the string.

const mask = (cc,num = 4,mask = '*') =>
   cc.slice(0, -num).replace(/./g, mask) + cc.slice(-num);

mask(1234567890) // '******7890'
mask(1234567890,3) // '*******890'
mask(1234567890,4,'$') // '$$$$$$7890'
mask(1234567890,-4,'$') // '1234$$$$$$'