✨ Add first and last functions

src/first.ts

@@ -0,0 +1,36 @@
+import { IsString } from '@/isString'
+import { InferArray } from '@/types'
+
+type first<T extends string | unknown[]> = IsString<T> extends true
+  ? string
+  : T extends never[]
+  ? undefined
+  : InferArray<T>
+
+/**
+ * Returns the first element of the given list or string.
+ *
+ * @param val - string or any array object
+ * @returns The first element of the `val`
+ *
+ * @example
+ * String
+ * first('hello') // 'h'
+ *
+ * @example
+ * Array
+ * first('hello', 'new', 'world') // 'hello'
+ * first([]) // undefined
+ * first(['one', 2, 3, 4]) // 'one'
+ */
+const first = <T extends string | unknown[]>(val: T): first<T> => {
+  if (Array.isArray(val)) {
+    return val.length
+      ? (val.slice(0, 1)[0] as first<T>)
+      : (undefined as first<T>)
+  } else {
+    return (val as string).slice(0, 1) as first<T>
+  }
+}
+
+export { first }

src/index.ts

@@ -1,5 +1,6 @@
 export { add } from '@/add'
 export { divide } from '@/divide'
+export { first } from '@/first'
 export type { IsBigint } from '@/isBigint'
 export { isBigint } from '@/isBigint'
 export type { IsNill } from '@/isNill'
@@ -12,6 +13,7 @@ export type { IsString } from '@/isString'
 export { isString } from '@/isString'
 export type { IsSymbol } from '@/isSymbol'
 export { isSymbol } from '@/isSymbol'
+export { last } from '@/last'
 export { multiply } from '@/multiply'
 export { startsWith } from '@/startsWith'
 export { subtract } from '@/subtract'

src/last.ts

@@ -0,0 +1,34 @@
+import { IsString } from '@/isString'
+import { InferArray } from '@/types'
+
+type Last<T extends string | unknown[]> = IsString<T> extends true
+  ? string
+  : T extends never[]
+  ? undefined
+  : InferArray<T>
+
+/**
+ * Returns the last element of the given list or string.
+ *
+ * @param val - string or any array object
+ * @returns The last element of the `val`
+ *
+ * @example
+ * String
+ * last('hello') // 'o'
+ *
+ * @example
+ * Array
+ * last('hello', 'new', 'world') // 'world'
+ * last([]) // undefined
+ * last(['one', 2, 3, 4]) // 4
+ */
+const last = <T extends string | unknown[]>(val: T): Last<T> => {
+  if (Array.isArray(val)) {
+    return val.length ? (val.slice(-1)[0] as Last<T>) : (undefined as Last<T>)
+  } else {
+    return (val as string).slice(-1) as Last<T>
+  }
+}
+
+export { last }

test/first.spec.ts

@@ -0,0 +1,34 @@
+import { first } from '@/first'
+
+describe('first', () => {
+  const table: [string | unknown[], unknown][] = [
+    ['', ''],
+    ['a', 'a'],
+    [' ab', ' '],
+    ['abcd', 'a'],
+    ['abcde ', 'a'],
+    ['abcd_', 'a'],
+    [[], undefined],
+    [[''], ''],
+    [['hello', 'world'], 'hello'],
+    [['hello', 'new', 'world'], 'hello'],
+    [
+      [['hello', 'new', 'world'], 'test'],
+      ['hello', 'new', 'world']
+    ],
+    [
+      [
+        ['hello', 'new', 'world'],
+        ['hello2', 'world']
+      ],
+      ['hello', 'new', 'world']
+    ],
+    [[0], 0],
+    [[0, 3, 6, 9], 0],
+    [[{}], {}]
+  ]
+
+  it.each(table)('first(%s) -> %s', (val, expected) => {
+    expect(first(val)).toEqual(expected)
+  })
+})

test/last.spec.ts

@@ -0,0 +1,31 @@
+import { last } from '@/last'
+
+describe('last', () => {
+  const table: [string | unknown[], unknown][] = [
+    ['', ''],
+    ['a', 'a'],
+    [' ab', 'b'],
+    ['abcd', 'd'],
+    ['abcde ', ' '],
+    ['abcd_', '_'],
+    [[], undefined],
+    [[''], ''],
+    [['hello', 'world'], 'world'],
+    [['hello', 'new', 'world'], 'world'],
+    [[['hello', 'new', 'world'], 'test'], 'test'],
+    [
+      [
+        ['hello', 'new', 'world'],
+        ['hello2', 'world']
+      ],
+      ['hello2', 'world']
+    ],
+    [[0], 0],
+    [[0, 3, 6, 9], 9],
+    [[{}], {}]
+  ]
+
+  it.each(table)('last(%s) -> %s', (val, expected) => {
+    expect(last(val)).toEqual(expected)
+  })
+})