feat: add string capitalize algorithm

String/Capitalize.js

@@ -0,0 +1,20 @@
+/**
+ * @function capitalize
+ * @description Capitalizes the first letter of a string and
+ *   lowercases the rest.
+ * @param {string} str - The input string
+ * @returns {string} - The capitalized string
+ * @see https://en.wikipedia.org/wiki/Capitalization
+ * @example capitalize("hello") => "Hello"
+ * @example capitalize("hELLO WORLD") => "Hello world"
+ * @example capitalize("") => ""
+ */
+const capitalize = (str) => {
+  if (typeof str !== 'string') {
+    throw new TypeError('Input must be a string')
+  }
+  if (str.length === 0) return ''
+  return str.charAt(0).toUpperCase() + str.slice(1).toLowerCase()
+}
+
+export { capitalize }

String/test/Capitalize.test.js

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+import { capitalize } from '../Capitalize.js'
+
+describe('capitalize', () => {
+  test("capitalize('hello') returns 'Hello'", () => {
+    expect(capitalize('hello')).toBe('Hello')
+  })
+
+  test("capitalize('hELLO WORLD') returns 'Hello world'", () => {
+    expect(capitalize('hELLO WORLD')).toBe('Hello world')
+  })
+
+  test("capitalize('') returns ''", () => {
+    expect(capitalize('')).toBe('')
+  })
+
+  test('capitalize single character', () => {
+    expect(capitalize('a')).toBe('A')
+  })
+
+  test('capitalize already capitalized string', () => {
+    expect(capitalize('Hello')).toBe('Hello')
+  })
+
+  test('capitalize all uppercase string', () => {
+    expect(capitalize('HELLO')).toBe('Hello')
+  })
+
+  test('capitalize string with numbers', () => {
+    expect(capitalize('1hello')).toBe('1hello')
+  })
+
+  test('capitalize string starting with space', () => {
+    expect(capitalize(' hello')).toBe(' hello')
+  })
+
+  test('throws TypeError for non-string input', () => {
+    expect(() => capitalize(123)).toThrow(TypeError)
+  })
+
+  test('throws TypeError for null input', () => {
+    expect(() => capitalize(null)).toThrow(TypeError)
+  })
+})