Count Occurrences of each Element in Array in JavaScript

Count Occurrences of each Element in Array #

To count the occurrences of each element in an array:

1. Declare a variable that stores an empty object.
2. Use the for...of loop to iterate over the array.
3. On each iteration, increment the count for the current element if it exists or initialize the count to 1.

Copied!
const arr = ['a', 'b', 'a', 'a', 'c', 'c'];

const count = {};

for (const element of arr) {
  if (count[element]) {
    count[element] += 1;
  } else {
    count[element] = 1;
  }
}

console.log(count); // 👉️ {a: 3, b: 1, c: 2}

We first declare a variable and set it to an empty object.

The for...of loop allows us to iterate over the array.

If the element is present in the object, we increment the corresponding value, otherwise we initialize the key to 1.

The keys in the object are the array elements, and the values are the occurrences of each element in the array.

Copied!
const arr = [1, 1, 1, 2, 3, 3];

const count = {};

for (const element of arr) {
  if (count[element]) {
    count[element] += 1;
  } else {
    count[element] = 1;
  }
}

console.log(count); // 👉️ {1: 3, 2: 1, 3: 2}

This seems a bit confusing as object keys can only be of type string or symbol in JavaScript.

However, you can still access the object's keys either way.

Copied!
console.log(count[1]); // 👉️ 3
console.log(count['1']); // 👉️ 3

You can achieve the same result by using a basic for loop.

Copied!
const arr = ['a', 'b', 'a', 'a', 'c', 'c'];

const count = {};

for (let index = 0; index < arr.length; index++) {
  const element = arr[index];

  if (count[element]) {
    count[element] += 1;
  } else {
    count[element] = 1;
  }
}

console.log(count); // 👉️ {a: 3, b: 1, c: 2}

This code snippet achieves the same goal. However, we use a basic for loop, instead of the for...of loop.

Further Reading #

• Count Elements in Array that match a Condition using JS
• Count the Duplicates in an Array using JavaScript
• Count the True values in an Array using JavaScript
• Count the Unique Elements in an Array in JavaScript
• Find the Index of all Occurrences of an Element in JS Array