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javascript/snippets/mask.md

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+---
+title: Mask a value
+type: snippet
+tags: [string]
+cover: rocky-beach-3
+dateModified: 2020-10-21T21:54:53+03:00
+---
+
+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.prototype.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.
+
+```js
+const mask = (cc, num = 4, mask = '*') =>
+  `${cc}`.slice(-num).padStart(`${cc}`.length, mask);
+```
+
+```js
+mask(1234567890); // '******7890'
+mask(1234567890, 3); // '*******890'
+mask(1234567890, -4, '$'); // '$$$$567890'
+```