How to Count Frequency of Array Items in JavaScript

JavaScript arrays are powerful data structures used by many web developers. They offer tons of functions and features. Often you may need to count frequency of array items in JavaScript. In this article, we will learn how to do this easily.

How to Count Frequency of Array Items in JavaScript

Let us say you have the following array.

var arr = [5, 5, 3, 2, 2, 2, 2, 2, 9, 4];

1. Using JavaScript

We will use a JavaScript object to store the counts of each element. We will store the array item as key and its count as the property.

var counts = {};

We will loop through the array items one by one and do one of the following:

1. If the item is not present as a key, we will add it to JS object counts.
2. If the item is already present in the JS object, we increment its count.

for (const num of arr) {
  counts[num] = counts[num] ? counts[num] + 1 : 1;
}
console.log(counts); //{'5':2,'3':1,'2':5,'9':1,'4':1}

2. Using Underscore or Lodash

If you are using third-party libraries like Underscore or Lodash, you can use count() function which is available directly out of the box.

_.countBy(array);

Here is an example.

_.countBy([5, 5, 3, 2, 2, 2, 2, 2, 9, 4])
=> Object {2: 5, 3: 1, 4: 1, 5: 3, 9: 1}

In this article, we have learnt how to get frequency count of array items using JavaScript.

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