How to use

string

function

in

fast-check

const nonEmpty = () =>
 fc.oneof(fc.string(), fc.integer(), fc.double(), fc.boolean())

describe('capitalize', () => {
 it('returns a string with its first letter capitalized', () => {
  expect(capitalize('margarita')).toBe('Margarita');
  expect(capitalize(capitalize('margarita'))).toBe('Margarita');
  expect(capitalize('Margarita')).toBe('Margarita');
 });

 it('returns a string of the same length', () => {
  fc.assert(
   fc.property(fc.string(), text => {
    expect(typeof capitalize(text)).toBe('string');
    expect(capitalize(text)).toHaveLength(text.length);
   }),
  );
 });

 it('returns the same result if applied twice', () => {
  fc.assert(
   fc.property(fc.string(), text => {
    expect(capitalize(capitalize(text))).toBe(capitalize(text));
   }),
  );
 });

 it('handles null and undefined values without crashing', () => {
  expect(capitalize(null)).toBeNull();
  expect(capitalize(undefined)).toBeNull();
 });
});