feat: Added fromatting for Credit Card Number

README.md

@@ -143,11 +143,11 @@ These changes improve throughput and reduce memory pressure when working with la
 - [formatFileSize](#formatfilesize) - Converts a number of bytes into a human-readable file size string (B, KB, MB, GB, TB).
 - [formatOrdinal](#formatordinal) -  Converts a number into its ordinal string representation (e.g., 1 → "1st", 2 → "2nd").
 - [formatList](#formatlist) - Formats an array of strings into a human-readable list with proper commas and "and".
+- [formatCreditCard](#formatcreditcard) - Formats a credit card number by grouping digits into readable parts.
 - [formatToOctal](#formattotoctal) - Converts a decimal number to octal, optional "0o" prefix.
 - [formatTemperature](#formattemperature) - Converts temperatures between Celsius, Fahrenheit, and Kelvin.
 - [formatToBinary](#formattobinary) - Converts a decimal integer to a binary string with optional bit grouping.
 - [formatToHexadecimal](#formattohexadecimal) - Converts temperatures between Celsius, Fahrenheit, and Kelvin.
-
 - [formatToDecimal](#formattodecimal) - Converts base-2/8/16 strings to decimal.
 
 ## 📋 API Reference
@@ -1486,6 +1486,27 @@ formatList(['apple', 123]);                     // TypeError
 | --------- | -------- | -------- | ----------------------------------------- |
 | arr       | string[] | required | The array of strings to format as a list. |
 
+#### <a id="formatcreditcard"></a>formatCreditCard(cardNumber)
+Formats a credit card number into readable groups of digits separated by spaces.
+Supports 15-digit (AmEx) and 16-digit (Visa/MasterCard) numbers.
+Non-digit characters are automatically stripped before formatting.
+Throws an error if the input is not a string.
+
+```javascript
+import { formatCreditCard } from 'stringzy';
+
+formatCreditCard('1234567812345678'); // "1234 5678 1234 5678"
+formatCreditCard('4111111111111111'); // "4111 1111 1111 1111"
+formatCreditCard('378282246310005');  // "3782 822463 10005" (AmEx)
+formatCreditCard('4111-1111-1111-1111'); // "4111 1111 1111 1111"
+formatCreditCard('123'); // "" (invalid length)
+formatCreditCard('');    // "" (empty string)
+```
+
+| Parameter  | Type   | Default  | Description                                                         |
+| ---------- | ------ | -------- | ------------------------------------------------------------------- |
+| cardNumber | string | required | The credit card number to format. Cannot include non-digit characters. |
+
 #### <a id="formattotoctal"></a>formatToOctal(num, options?)
 
 Converts a decimal number to its octal (base-8) string representation. Supports negatives and an optional `0o` prefix.

src/formatting/creditCard.ts

@@ -0,0 +1,29 @@
+/**
+ * Formats a credit card number by grouping digits into readable parts.
+ *
+ * @param {string} cardNumber - The credit card number to format.
+ * @returns {string} The formatted credit card number.
+ * @throws {TypeError} If the input is not a string.
+ */
+export function formatCreditCard(cardNumber: string): string {
+  if (typeof cardNumber !== 'string') {
+    throw new TypeError('Input must be a string');
+  }
+
+  // Remove all non-digit characters
+  const cleaned = cardNumber.replace(/\D/g, '');
+
+  // Only accept 15 or 16 digit card numbers
+  if (cleaned.length !== 15 && cleaned.length !== 16) {
+    return '';
+  }
+
+  // Format based on length:
+  // 16 digits → 4-4-4-4 (Visa, MasterCard)
+  if (cleaned.length === 16) {
+    return cleaned.replace(/(\d{4})(?=\d)/g, '$1 ').trim();
+  }
+
+  // 15 digits → 4-6-5 (AmEx)
+  return cleaned.replace(/(\d{4})(\d{6})(\d{5})/, '$1 $2 $3');
+}

src/formatting/index.ts

@@ -8,6 +8,7 @@ export { formatPercentage } from './percentage';
 export { formatFileSize } from './fileSize';
 export { formatOrdinal } from './ordinal';
 export { formatList } from './listToString';
+export { formatCreditCard } from './creditCard';
 export { formatToOctal } from './octal';
 export { formatTemperature } from './temperature';
 export { formatToBinary } from './binary';
@@ -24,6 +25,7 @@ import { formatPercentage } from './percentage';
 import { formatFileSize } from './fileSize';
 import { formatOrdinal } from './ordinal';
 import { formatList } from './listToString';
+import { formatCreditCard } from './creditCard';
 import { formatToOctal } from './octal';
 import { formatTemperature } from './temperature';
 import { formatToBinary } from './binary';
@@ -41,6 +43,7 @@ export const formatting = {
   formatFileSize,
   formatOrdinal,
   formatList,
+  formatCreditCard,
   formatTemperature,
   formatToBinary,
   formatToHexadecimal,

src/tests/formatting/creditCard.test.ts

@@ -0,0 +1,31 @@
+import { describe, it } from 'node:test';
+import assert from 'node:assert';
+import { formatCreditCard } from '../../formatting/creditCard';
+
+describe('formatCreditCard', () => {
+  it('formats 16-digit card numbers correctly', () => {
+    assert.strictEqual(formatCreditCard('1234567812345678'), '1234 5678 1234 5678');
+    assert.strictEqual(formatCreditCard('4111111111111111'), '4111 1111 1111 1111');
+  });
+
+  it('formats 15-digit card numbers (AmEx) correctly', () => {
+    assert.strictEqual(formatCreditCard('378282246310005'), '3782 822463 10005');
+  });
+
+  it('removes non-digit characters before formatting', () => {
+    assert.strictEqual(formatCreditCard('4111-1111-1111-1111'), '4111 1111 1111 1111');
+    assert.strictEqual(formatCreditCard('3782 8224 6310 005'), '3782 822463 10005');
+  });
+
+  it('returns empty string for invalid lengths', () => {
+    assert.strictEqual(formatCreditCard('123'), '');
+    assert.strictEqual(formatCreditCard('11112222333344445555'), '');
+    assert.strictEqual(formatCreditCard(''), '');
+  });
+
+  it('throws an error if input is not a string', () => {
+    assert.throws(() => formatCreditCard(1234567812345678 as any), /Input must be a string/);
+    assert.throws(() => formatCreditCard(null as any), /Input must be a string/);
+    assert.throws(() => formatCreditCard(undefined as any), /Input must be a string/);
+  });
+});