Write a JavaScript program to create a new string from a given string changing the position of first and last characters. The string length must be greater than or equal to 1

function first_last(str1) 
  {
  if (str1.length <= 1)
  {
    return str1;
  }
  mid_char = str1.substring(1, str1.length - 1);
  return (str1.charAt(str1.length - 1)) + mid_char + str1.charAt(0);
}
console.log(first_last('a'));
console.log(first_last('ab'));
console.log(first_last('abc'));