function* generateSequence(start, end) {
  for (let i = start; i <= end; i++) yield i;
}

function* generateAlphaNum() {

  // 0..9
  yield* generateSequence(48, 57);

  // A..Z
  yield* generateSequence(65, 90);

  // a..z
  yield* generateSequence(97, 122);

}

let str = '';

for(let code of generateAlphaNum()) {
  str += String.fromCharCode(code);
}

alert(str); // 0..9A..Za..z