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. Create an object that will store each array element as the key and its number of occurrences as the value.
2. Use the for...of loop to iterate over the array.
3. On each iteration, increment the count for the specific element if it exists in the object or set the count of the element to 1 if it doesn't.

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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.

Our if condition checks if we already encountered a specific array element and initialized it on the object or not.

If it's the first time we encounter an array element, we set the element as the object's key and initialize its value to 1.

If we encounter the same element again, we increment the value of the corresponding key in the object.

The final result is an object, whose keys are the array elements and the values - their occurrences in the array.

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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.

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

If you try to access the key of the object using a number, it will automatically get converted to a string under the hood.

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// Supported in IE

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 #

• Get Multiple Random Elements from an Array in JavaScript
• Add Array of Values to an Existing Set in JavaScript
• Create a Map from an Array using JavaScript
• Check if a Value is NOT in an Array using JavaScript
• How to Flatten an Array using JavaScript
• Convert an Array to JSON using JavaScript
• Create an Object from Two Arrays in JavaScript
• Convert an Object to an Array of Objects in JavaScript
• Convert an Array of Objects to a Map in JavaScript
• Get every Nth Element of Array using JavaScript
• Get the Second to Last Element in Array in JavaScript
• Array.push() Element if does not exist using JavaScript