# Mask the first characters of a string

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

Canonical: https://1loc.dev/string/replace-the-first-given-number-of-characters-of-a-string-with-another-character/
Version: 1.0.0
Runtime: JavaScript · Node.js 22 / modern browsers
Verification: Local fixtures; no external services
Updated: 2026-10-28

## Implementation

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

## Input

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

## Expected output

```json
"••••••••••••4242"
```

## Edge case

Input:
```json
{
  "text": "abc",
  "count": 5,
  "mask": "*"
}
```
Output:
```json
"***"
```

## Limits

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

The lab validates types and size before calling this function. Add equivalent validation at your own trust boundary.

## Source and license

Original: https://github.com/phuocng/1loc/blob/14b481cf60dff1620ab1bb99a43b1f68068cdd9b/contents/replace-the-first-given-number-of-characters-of-a-string-with-another-character.mdx
Author: Nguyen Huu Phuoc and original contributors
Contributors: See original repository history
Source commit: 14b481cf60dff1620ab1bb99a43b1f68068cdd9b
Copyright (c) 2020 phuoc-ng
License: MIT — https://1loc.dev/licenses/
Maintainer: Oveyon
Modifications: Curated implementation; explicit input contract, edge case and limits. Original MDX retained as inert source evidence.