Group Elements By Count

Introduction

In this lab, we will learn about the countBy function, which is used to group the elements of an array based on the given function and returns the count of elements in each group. We will explore how this function can be used to map values of an array to a function or property name and how to create an object to count the elements in each group using Array.prototype.reduce(). We will also look at several examples to understand how to use this function in real-world scenarios.

How to Group and Count Elements in an Array Using JavaScript

To group and count elements in an array using JavaScript, follow these steps:

1. Open the Terminal/SSH and type node to start practicing coding.
2. Use the Array.prototype.map() method to map the values of an array to a function or property name.
3. Use the Array.prototype.reduce() method to create an object where the keys are produced from the mapped results.
4. Create a function called countBy that takes an array and a function as arguments.
5. Inside the countBy function, use a ternary operator to check if the argument passed is a function or a property name. If it's a function, use it as the mapping function. If it's a property name, access that property of the array elements.
6. Use the reduce() method to create an object where each key represents a unique element in the array and its value is the number of times it appears in the array.

Here's the code:

const countBy = (arr, fn) =>
  arr
    .map(typeof fn === "function" ? fn : (val) => val[fn])
    .reduce((acc, val) => {
      acc[val] = (acc[val] || 0) + 1;
      return acc;
    }, {});

You can test the countBy function with the following examples:

countBy([6.1, 4.2, 6.3], Math.floor); // {4: 1, 6: 2}
countBy(["one", "two", "three"], "length"); // {3: 2, 5: 1}
countBy([{ count: 5 }, { count: 10 }, { count: 5 }], (x) => x.count); // {5: 2, 10: 1}

Summary

Congratulations! You have completed the Count Grouped Elements lab. You can practice more labs in LabEx to improve your skills.