javascript string handling with array why it's print 'r' on console but not 'z'

javascript string handling with array why it's print 'r' on console but
not 'z'

var cun = function(cun){
cun[0] = 'z';
console.log(cun[0]);
return cun;
}
cun("ratul");
Why it is print r on console but not z ? why i can't change the string
using array notation?