JavaScript: Counting Same Occurrences in an Array

Hi!

For example, we have an array:

[2, "tea", 2, 1, 2, "tea", "bag", "bag", "bag", 1]

And we want to count the occurrence of every element in the array — how many times it appears.

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Method

We can use Object construction method to achieve that.

The steps:

1. Create an empty object.
2. Loop through the input array.
3. While looping — for each of the array element — create an object property (key) of that.
4. The value of that object property (key) will be an array of index(es) where the input element is found.

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Let's Go

It goes as such:

function occurrence(array) { var result = {}; if (array instanceof Array) { // Check if input is array. array.forEach(function (v, i) { if (!result[v]) { // Initial object property (key) creation. result[v] = [i]; // Assign an array for that property. } else { // Same occurrences found. result[v].push(i); // Fill the array. } }); } return result; }

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Usage

Let's take the example array above as the input. So then:

var array = [2, "tea", 2, 1, 2, "tea", "bag", "bag", "bag", 1]; occurrence(array); // This will generate the output.

The output is an Object which consists of:

{ array_element_0: [/*array of index(es)*/], array_element_1: [/*array of index(es)*/], array_element_2: [/*array of index(es)*/], // And so forth. }

If you try that snippet with the input array in the JavaScript console, you'll be seeing something like this (expand it):

To get the length for each of the property, you can do like this:

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Digest

The function example above will generate an object consists of elements of the array input as the object property and the array of index(es) where each was found as the value of the object property.

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Additional — Transformed Output

function occurrence(array) { var result = {}; if (array instanceof Array) { array.forEach(function (v, i) { if (!result[v]) { result[v] = [i]; } else { result[v].push(i); } }); Object.keys(result).forEach(function (v) { result[v] = { index: result[v], length: result[v].length, }; }); } return result; }

This additional snippet will output:

{ array_element_0: { index: [/*array of index(es)*/], length: /*array of index(es) length — integer.*/ }, array_element_1: { index: [/*array of index(es)*/], length: /*array of index(es) length — integer.*/ }, // And so forth. }

To access the array of index(es) of a particular element, do this:

var index_tea = occurrence(array)["tea"].index;

And to access the length of a particular element:

var length_tea = occurrence(array)["tea"].length;

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Cheers! 👋