first non repeating character javascript

function firstNotRepeatingCharacter(s) {
    for (let i = 0; i < s.length; i++) {
        if(s.indexOf(s.charAt(i)) == s.lastIndexOf(s.charAt(i))) {
            return s.charAt(i)
        }
    }
    return '_'    
}
function firstRepeatingCharacter(str) {
  for (let i = 0; i < str.length; i++) {
    if (str.indexOf(str.charAt(i)) !== str.lastIndexOf(str.charAt(i))) {
      return str.charAt(i)
    }
  }
  return 'no results found'
}