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Lodash _.countBy


The lodash _.countBy method is a powerful utility that helps you count the number of occurrences of values in a collection or an array in JavaScript. It returns an object where the keys are the values in the collection, and the values are the number of occurrences of that value in the collection.

This method takes two parameters: the collection to iterate over and a function that maps each element to its key. The function is optional, and if none is provided, _.identity is used as the default mapping function.

The _.countBy method works by iterating over each element in the collection, applying the mapping function to each, and using the result as the key for the output object. If the key already exists in the output object, the corresponding value is incremented; otherwise, it is initialized to 1.

The following is an example of how to use the method:

const _ = require('lodash');

const pets = ['dog', 'cat', 'bird', 'dog', 'cat'];

const petCount = _.countBy(pets);


// Output: { dog: 2, cat: 2, bird: 1 }

In this example, the _.countBy method is used to count the number of occurrences of each pet in the array. The resulting object shows that there are two dogs, two cats, and one bird in the array.

Another example:

const _ = require('lodash');

const users = [
  { name: 'John', role: 'admin' },
  { name: 'Jane', role: 'user' },
  { name: 'Adam', role: 'admin' },
  { name: 'Eva', role: 'user' },
  { name: 'Mike', role: 'user' },
  { name: 'Susan', role: 'admin' },

const roleCount = _.countBy(users, 'role');


// Output: { admin: 3, user: 3 }

In this example, the _.countBy method is used to count the number of occurrences of each role in the array of users. The resulting object shows that there are three admins and three users in the array.

In conclusion, the _.countBy method is an essential utility that makes it easy to count the number of occurrences of values in a collection or an array in JavaScript. It can be used in a variety of scenarios where counting is required, such as data analysis, reporting, and more.