repeated letter find in string javascript code example

Example 1: first repeated character in a string javascript

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'
}

Example 2: duplicate characters in a string javascript

const text = 'abcda'.split('')
text.some( (v, i, a) => {
  return a.lastIndexOf(v) !== i
}) // true