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

mask

Replaces all but the last num of characters with the specified mask character.

Use String.slice() to grab the portion of the characters that need to be masked and use String.replace() with a regexp to replace every character with the mask character. Concatenate the masked characters with the remaining unmasked portion of the string. Omit the second argument, num, to keep a default of 4 characters unmasked. If num is negative, the unmasked characters will be at the start of the string. Omit the third argument, mask, to use a default character of '*' for the mask.

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, '$'); // '1234$$$$$$'