eolas/Data_Structures/Patterns/Frequency_counters.md

---
tags:
  - Data_Structures
  - patterns
---

# Frequency counters

Here is a concise method for counting the number instances of each type of array element.

```js
function frequencyCounter(...inputArr) {
    let counter = {};
     inputArr.forEach((item) => {
    // If item doesn't exist as a key in the counter, make 0 the val and add one
    // If item already exists as key in the counter, increment it by one
    counter[item] = (counter[item] || 0) + 1
  });  
  return counter; 
}
```

An application of this would be checking for duplicates in an array:

```js
const testArr = [1, 4, 3, 3];
const arrayFreq = frequencyCounter(testArr);
let count = Object.values(arrayFreq)

if (count.some((ele) => ele > 1)) {
    return 'There are duplicates';
} else return 'No duplicates'; 
```