When to use 'i' vs. 'str[i]'?

why does using 'i' in the if statement return different results than using 'str[i]'?

function ExOh(str) { 
  xCount = 0;
  oCount = 0;
  for (var i=0;i<str.length;i++){
    if (str[i]=='x') {
      xCount++;
    } else if (str[i]=='o') {
      oCount++;
    }
  }
  if (xCount==oCount) {
    return true;
  } else {
    return false;
  }
}

console.log(ExOh("xox"));