At least validator with test

atLeast.js

/**
 * Array must containt at least nessesary values
 * @param  {Number} [count=1] nessesary values in array
 * @param  {String} [message='You must select at least one of the options'] error message
 * @return {?String}
 */
const atLeast = (
  count = 1,
  message = 'You must select at least one of the options') =>
  (value) => {
    return value.length >= count ? undefined : message;
  };

export default atLeast;

tests\atLeast.spec.js

import createValidator from '../atLeast';

describe('At least validator', () => {
  test('must return undefined when validation is passed', () => {
    const atLeast = createValidator();
    expect(atLeast(['one', 'ring', 'to', 'rule', 'them', 'all'])).toBeUndefined();
  });

  test('must return error message when validation is faled', () => {
    const atLeast = createValidator();
    expect(atLeast([])).not.toBeUndefined();
  });

  test('must return special error message when validation is faled', () => {
    const message = 'Fatal error';
    const atLeastWithMessage = createValidator(1, message);
    expect(atLeastWithMessage([])).toBe(message);
  });

  test('with another at least value when validation is passed', () => {
    const atLeastFive = createValidator(5);
    expect(atLeastFive(['one', 'ring', 'to', 'rule', 'them', 'all'])).toBeUndefined();
  });

  test('with another at least value when validation is failed', () => {
    const atLeastFive = createValidator(10);
    expect(atLeastFive(['one', 'ring', 'to', 'rule', 'them', 'all'])).not.toBeUndefined();
  });
});