A generator that works something like Ruby's `String#succ` (for lowercase strings only)

string-succ.test.js

// Run with jest

const succ = function * (string) {
  const matchResult = string.match(/^([a-z]*)([a-z])$/);
  if (matchResult) {
    const prefix = matchResult[1];
    const last_char = matchResult[2];

    if (last_char == 'z') {
      yield arguments.callee(prefix).next().value + 'a';
    }
    else {
      yield (prefix + String.fromCharCode(last_char.charCodeAt(0) + 1));
    }
  } else {
    if (string.match(/^[a-z]*$/)) {
      yield 'a';
    }
    else {
      throw new TypeError('String must include only lowercase letters, or be empty');
    }
  }
}

test('', () => {
  const subject = succ('');
  expect(subject.next().value).toEqual('a');
});

test('a', () => {
  const subject = succ('a');
  expect(subject.next().value).toEqual('b');
});

test('z', () => {
  const subject = succ('z');
  expect(subject.next().value).toEqual('aa');
});

test('aa', () => {
  const subject = succ('aa');
  expect(subject.next().value).toEqual('ab');
});

test('az', () => {
  const subject = succ('az');
  expect(subject.next().value).toEqual('ba');
});

test('zz', () => {
  const subject = succ('zz');
  expect(subject.next().value).toEqual('aaa');
});

test('WHAT', () => {
  const subject = succ('WHAT');
  expect(() => subject.next()).toThrow(TypeError);
});