1let strArray = [ "q", "w", "w", "w", "e", "i", "u", "r"];
2let findDuplicates = arr => arr.filter((item, index) => arr.indexOf(item) != index)
3
4console.log(findDuplicates(strArray)) // All duplicates
5console.log([...new Set(findDuplicates(strArray))]) // Unique duplicates

1function findDuplicates(arr) {
2	const duplicates = new Set()
3  
4  return arr.filter(item => {
5  	if (duplicates.has(item)) {
6    	return true
7    }
8    duplicates.add(item)
9    return false
10  })
11}

1// JavaScript - finds if there is duplicate in an array. 
2// Returns True or False.
3
4const isThereADuplicate = function(arrayOfNumbers) {
5    // Create an empty associative array or hash. 
6    // This is preferred,
7    let counts = {};
8    // // but this also works. Comment in below and comment out above if you want to try.
9    // let counts = [];
10
11    for(var i = 0; i <= arrayOfNumbers.length; i++) {
12        // As the arrayOfNumbers is being iterated through,
13        // the counts hash is being populated.
14        // Each value in the array becomes a key in the hash. 
15        // The value assignment of 1, is there to complete the hash structure.
16        // Once the key exists, meaning there is a duplicate, return true.
17        // If there are no duplicates, the if block completes and returns false.
18        if(counts[arrayOfNumbers[i]] === undefined) {
19            counts[arrayOfNumbers[i]] = 1;
20        } else {
21            return true;
22        }
23    }
24    return false;
25}

1const names = ['Mike', 'Matt', 'Nancy', 'Adam', 'Jenny', 'Nancy', 'Carl']
2
3const count = names =>
4  names.reduce((a, b) => ({ ...a,
5    [b]: (a[b] || 0) + 1
6  }), {}) // don't forget to initialize the accumulator
7
8const duplicates = dict =>
9  Object.keys(dict).filter((a) => dict[a] > 1)
10
11console.log(count(names)) // { Mike: 1, Matt: 1, Nancy: 2, Adam: 1, Jenny: 1, Carl: 1 }
12console.log(duplicates(count(names))) // [ 'Nancy' ]

1const findDuplicates = (arr) => {
2  let sorted_arr = arr.slice().sort(); // You can define the comparing function here. 
3  // JS by default uses a crappy string compare.
4  // (we use slice to clone the array so the
5  // original array won't be modified)
6  let results = [];
7  for (let i = 0; i < sorted_arr.length - 1; i++) {
8    if (sorted_arr[i + 1] == sorted_arr[i]) {
9      results.push(sorted_arr[i]);
10    }
11  }
12  return results;
13}
14
15let duplicatedArray = [9, 4, 111, 2, 3, 4, 9, 5, 7];
16console.log(`The duplicates in ${duplicatedArray} are ${findDuplicates(duplicatedArray)}`);

1const findDuplicates = (arr) => {
2  let sorted_arr = arr.slice().sort(); // You can define the comparing function here. 
3  // JS by default uses a crappy string compare.
4  // (we use slice to clone the array so the
5  // original array won't be modified)
6  let results = [];
7  for (let i = 0; i < sorted_arr.length - 1; i++) {
8    if (sorted_arr[i + 1] == sorted_arr[i]) {
9      results.push(sorted_arr[i]);
10    }
11  }
12  return results;
13}
14
15let duplicatedArray = [9, 4, 111, 2, 3, 9, 4, 5, 7];
16console.log(`The duplicates in ${duplicatedArray} are ${findDuplicates(duplicatedArray)}`);