Mask the first characters of a string

Replace the first count characters with a mask character, keeping the length.

JavaScript Version 1.0.0 · MIT · .md

function maskStart({ text, count, mask }) { return text.slice(count).padStart(text.length, mask); }

Input & expected output

{
  "text": "4242424242424242",
  "count": 12,
  "mask": "•"
}

"••••••••••••4242"

The edge case

{
  "text": "abc",
  "count": 5,
  "mask": "*"
}

"***"

Know the limits

• Masking for display is not tokenization; never store or log the original value.

The function above is the implementation used by the lab. The lab validates JSON types and caps size before execution; include equivalent validation when accepting untrusted input in your application.

Keep exploring

• Capitalize a string — Uppercase the first Unicode code point and preserve the remaining text.
• Reverse a string — Reverse Unicode code points without splitting surrogate pairs.
• Check a palindrome — Compare a string with its code-point reversal.