capitalize

1/**
2 * Capitalizes the first letter of a string.
3 * If 'strict' is true, lowercases the rest of the string.
4 *
5 * @param str - The input string.
6 * @param strict - Whether to lowercase the rest of the string (default: false).
7 * @returns A new string with the first letter capitalized.
8 */
9export function capitalize(str: string, strict = false): string {
10  if (!str) return '';
11  const first = str[0].toUpperCase();
12  const rest = strict ? str.slice(1).toLowerCase() : str.slice(1);
13  return first + rest;
14}

1test('capitalize - default behavior (strict = false)', () => {
2  expect(capitalize('hello')).toBe('Hello');
3  expect(capitalize('hELLo')).toBe('HELLo');
4});
5
6test('capitalize - strict = true', () => {
7  expect(capitalize('hello', true)).toBe('Hello');
8  expect(capitalize('hELLo', true)).toBe('Hello');
9});
10
11test('capitalize - already capitalized', () => {
12  expect(capitalize('Hello')).toBe('Hello');
13});
14
15test('capitalize - single lowercase letter', () => {
16  expect(capitalize('a')).toBe('A');
17});
18
19test('capitalize - single uppercase letter', () => {
20  expect(capitalize('A')).toBe('A');
21});
22
23test('capitalize - empty string', () => {
24  expect(capitalize('')).toBe('');
25});
26
27test('capitalize - non-letter characters', () => {
28  expect(capitalize('123abc')).toBe('123abc');
29  expect(capitalize('!hello', true)).toBe('!hello');
30});