Javascript implementation to obtain string hash value_javascript skills

var I64BIT_TABLE =
 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_-'.split('');
 
function hash(input){
 var hash = 5381;
 var i = input.length - 1;
 
 if(typeof input == 'string'){
  for (; i > -1; i--)
   hash += (hash << 5) + input.charCodeAt(i);
 }
 else{
  for (; i > -1; i--)
   hash += (hash << 5) + input[i];
 }
 var value = hash & 0x7FFFFFFF;
 
 var retValue = '';
 do{
  retValue += I64BIT_TABLE[value & 0x3F];
 }
 while(value >>= 6);
 
 return retValue;
}