Find duplicates inside an array.

duplicates.js

const duplicates = input => {
  if (!Array.isArray(input)) return [];

  const workingCopy = [...input];
  const duplicatesSet = new Set();

  // length of 1: no duplicate or already found
  while (workingCopy.length > 1) {
    const element = workingCopy.shift();
    if (workingCopy.includes(element)) {
      duplicatesSet.add(element);
    }
  }
  return Array.from(duplicatesSet);
}

// example
console.log(duplicates(['a', 'b', 'a', 'c', 'c', 'c']));  // ['a', 'c']

duplicatesRec.js

const duplicates = (input, duplicatesSet = new Set()) => {
  if (!Array.isArray(input)) return [];
  
  const [element, ...rest] = input;
  if (rest.includes(element)) {
    duplicatesSet.add(element);
  }
  if (rest.length) return duplicates(rest, duplicatesSet);
  
  return Array.from(duplicatesSet);
}

// example
console.log(duplicates(['a', 'b', 'a', 'c', 'c', 'c'])); // ['a', 'c']

duplicatesWithSort.js

const duplicates = input => {
  if (!Array.isArray(input)) return [];
  
  const sortedInput = [...input].sort();
  const result = [];
  
  let counter = 0;
  while (counter < sortedInput.length - 1) {
    const currentElement = sortedInput[counter];
    let inc = 1;

    if (currentElement === sortedInput[counter + inc]) {
      result.push(sortedInput[counter]);
      while(currentElement === sortedInput[counter + inc]) {
      	inc++;
      }
    }
    
    counter = counter + inc;
  }
  
  return result;
}

// example
console.log(duplicates(['a', 'b', 'a', 'c', 'c', 'c'])); // ['a', 'c']