LeftPad in Javascript

function leftPad (str, len, ch) {
  if (!ch) {
    ch = '.';
  }
  
  str = String(str);
  if (ch.length !== 1) {
    throw 'Invalid Input'
  }
  
  len = len - str.length;
  strCh = '';
  for (i = 0; i < len; ++i) {
    strCh += ch;
  }
  
  return (strCh + str);
}
console.log(leftPad('1', 1));
console.log(leftPad('2', 2));
console.log(leftPad('3', 3));
console.log(leftPad('4', 4));
console.log(leftPad('5', 5));
console.log(leftPad('hello', 7));
console.log(leftPad("foo",  6));
console.log(leftPad("foo", 3));
console.log(leftPad("foobar", 3));
console.log(leftPad("foo", 6, "?"));