We cant replace the first character, because strings in JavaScript are immutable.

But we can make a new string based on the existing one, with the uppercased first character:

let newStr = str[0].toUpperCase() + str.slice(1);

Theres a small problem though. If str is empty, then str[0] is undefined, and as undefined doesnt have the toUpperCase() method, well get an error.

The easiest way out is to add a test for an empty string, like this:

function ucFirst(str) {
  if (!str) return str;

  return str[0].toUpperCase() + str.slice(1);
}

alert( ucFirst("john") ); // John

Open the solution with tests in a sandbox.