find-duplicates-in-array.js

// find duplicate primitive elements in an array (e.g. array of ints)
function findDuplicates(data) {

  let result = [];

  data.forEach(function(element, index) {
    
    // Find if there is a duplicate or not
    if (data.includes(element, index + 1)) {
      
      // Find if the element is already in the result array or not
      if (!result.includes(element)) {
        result.push(element);
      }
    }
  });

  return result;
}