Find Duplicates in String with JS reduce()

Posted by aut0maat10 on July 4, 2018

Las week, I wrote about removing duplicates from a string with JS methods filter() and indexOf(). But what if you wanted to return duplicate characters instead of removing them? After some more research, I learned a little trick with another JS method, i.e. reduce() .

According to the MDN docs, the reduce() method “applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value”.

That means you can do this to sum all numbers in an array:

const numbers = [1, 2, 3, 4];
const reduced = numbers.reduce((accumulator, currentValue) => accumulator + currentValue)
console.log(reduced) // => 10

The reduce() method also takes additional arguments, including an initial value. If you wanted to add an initial value of 5 to the previous example, you could do this:

const numbers = [1, 2, 3, 4];
const reduced = numbers.reduce((accumulator, currentValue) => accumulator + currentValue, 5)
console.log(reduced) // => 15

reduce() also takes arguments for the index of the current element, as well as the array you are applying the method to. The full list of arguments looks like this:

array.reduce(function(accumulator, currentValue, currentIndex, arr), initialValue)

In our case, we want to find and return duplicates in a string. First, we split our input string into an array, and call reduce() on that array. The trick here is setting the initialValue of reduce() to an array (line 7 in the example below), and then we can push duplicates to that array.

The callback also takes parameters for currentElement, currentIndex, and arr (which in this case refers to the input string split into an array of characters). Next, we use indexOf() to compare for duplicates, and if we get an indication of duplicates, we push the character to the duplicatesArr.

To avoid duplicates in the duplicatesArray, we also check for the current element there (indexOf() returns -1 if current element is not found). The final code looks like this:

const returnDuplicates = (input) => {
  const duplicates = input.split('').reduce((duplicatesArr, currentElement, currentIndex, arr) => {
    if (arr.indexOf(currentElement) !== currentIndex && duplicatesArr.indexOf(currentElement) === -1) {
    return duplicatesArr;
}, []);
  return duplicates.join('');

returnDuplicates('aaabbbcccdeffffff'); // => abcf


MDN docs: Array.prototype.reduce()

MDN docs: Array.prototype.indexOf()