有人可以解释这行代码吗？ 0 % 2

Question

Was working through the following problem:

function padIt accept 2 parameters:

1. str , it's a string representing the string to pad, we need to pad some "*" at leftside or rightside of str

2. n , it's a number, how many times to pad the string.

This is the answer:

function padIt(str,n){
  var count = 0;
  while ( count < n ) {
    count % 2 ? str += '*' : str = '*' + str;
    count ++
  }
  return str;
}

Can someone explain this part? count % 2 ? str += '*' : str = '*' + str;

Say I put these parameters in the function, padIt('a', 1) The first loop will have count = 0 so the function will have 0 % 2 which = 0 . Why does the function choose the option str = '*' + str to output '*a' ? Why not str += '*' to output 'a*' ?

Answer 1

This is a one-line shorthand for an if-else statement. It's called the conditional(ternary) operator.

 function padIt(str,n){
  var count = 0;
  while ( count < n ) {
    count % 2 ? str += '*' : str = '*' + str;
    count ++
   }
    return str;
  }

This is construct count % 2 ? str += '*' : str = '*' + str; count % 2 ? str += '*' : str = '*' + str; is the same as

      if(count % 2){
          str += '*';
       }else{

         str = '*' + str;
       }

count % 2 ? str += '*' : str = '*' + str; means if count is even do this if not do that.
The obvious advantage is that is it's shorter.