✨ Add toLower and toUpper functions

src/index.ts

@@ -14,4 +14,6 @@ export type { IsSymbol } from '@/isSymbol'
 export { isSymbol } from '@/isSymbol'
 export { multiply } from '@/multiply'
 export { subtract } from '@/subtract'
+export { toLower } from '@/toLower'
+export { toUpper } from '@/toUpper'
 export type { AnyFn } from '@/types'

src/toLower.ts

@@ -0,0 +1,13 @@
+/**
+ * Return lowercase string
+ *
+ * @param val - input string value
+ * @returns lowercase string
+ *
+ * @example
+ * toLower('Hello') // hello
+ */
+const toLower = <T extends string>(val: T): Lowercase<T> =>
+  val.toLowerCase() as Lowercase<T>
+
+export { toLower }

src/toUpper.ts

@@ -0,0 +1,13 @@
+/**
+ * Return uppercase string
+ *
+ * @param val - input string value
+ * @returns uppercase string
+ *
+ * @example
+ * toUpper('Hello') // HELLO
+ */
+const toUpper = <T extends string>(val: T): Uppercase<T> =>
+  val.toUpperCase() as Uppercase<T>
+
+export { toUpper }

test/toLower.spec.ts

@@ -0,0 +1,15 @@
+import { toLower } from '@/toLower'
+
+describe('toLower', () => {
+  const table: [string, string][] = [
+    ['', ''],
+    ['Hello', 'hello'],
+    ['HeLlo', 'hello'],
+    ['hello', 'hello'],
+    ['Hello World', 'hello world']
+  ]
+
+  it.each(table)('toLower(%s) -> %s', (val, expected) => {
+    expect(toLower(val)).toBe(expected)
+  })
+})

test/toUpper.spec.ts

@@ -0,0 +1,14 @@
+import { toUpper } from '@/toUpper'
+
+describe('toUpper', () => {
+  const table: [string, string][] = [
+    ['', ''],
+    ['Hello', 'HELLO'],
+    ['hello', 'HELLO'],
+    ['Hello World', 'HELLO WORLD']
+  ]
+
+  it.each(table)('toUpper(%s) -> %s', (val, expected) => {
+    expect(toUpper(val)).toBe(expected)
+  })
+})