mask

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

• Use String.prototype.slice() to grab the portion of the characters that will remain unmasked.
• Use String.padStart() to fill the beginning of the string with the mask character up to the original length.
• If num is negative, the unmasked characters will be at the start of the string.
• Omit the second argument, num, to keep a default of 4 characters unmasked.
• Omit the third argument, mask, to use a default character of '*' for the mask.

const mask = (cc, num = 4, mask = '*') =>
  `${cc}`.slice(-num).padStart(`${cc}`.length, mask);

mask(1234567890); // '******7890'
mask(1234567890, 3); // '*******890'
mask(1234567890, -4, '$'); // '$$$$567890'