A good string transcoding and decoding function (self-write)

function isString(variable) {  return Object.prototype.toString.call(variable).indexOf('String') != -1;}function isNumeric(variable) {  return !isNaN(parseFloat(variable)) && isFinite(variable);}function stringEncode(string) {  string = isString(string) || isNumeric(string) ? String(string) : '';  var code,    i = 0,    code_string = '',    len = string.length;  while(i < string.length) {    code = string.charCodeAt(i);    code_string += '' + String(code).length + code;    i++;  }  return code_string;}function stringDecode(code) {  var i = 0,    code_len,    decode_string = '';  code = String(code);  while(i < code.length) {    code_len = +code.charAt(i);    i++;    decode_string += String.fromCharCode(+code.substr(i, code_len));    i += code_len;  }  return decode_string;}