Check for duplicate chars, if duplicated replace with ')' else '('

duplicateEncoder.js

const check = (char, ind, word) => ~word.slice(0, ind).indexOf(char) 
  || ~word.slice(ind + 1).indexOf(char) ? ')' : '(';

function duplicateEncode(word) {
	return word
  	.toLowerCase()
    .split('')
    .map(check)
    .join('');
}