JS String Mask

// stringMask: 字符串掩码函数
// - str: 要处理的字符串
// - ch:  掩码字符
// - len: 尾部保留的明文长度
function stringMask(str, ch, len) {
  var strarr = Array.from(str);
  strarr[0] = 'a'
  if(len > str.length) {
    len = str.length;
  }
  if(len < 0) {
    len = 0;
  }
  for(var i = 0; i < str.length - len; ++i) {
    strarr[i] = ch;
  }
  return strarr.join('')
}

var s = "djofjaosgjoasdfajgoanfo1234";

msk = stringMask(s, '*', -1)
console.log(msk)

msk = stringMask(s, '*', 0)
console.log(msk)

msk = stringMask(s, '*', 1)
console.log(msk)

msk = stringMask(s, '*', s.length)
console.log(msk)

msk = stringMask(s, '*', s.length + 1)
console.log(msk)