fastify-riotjs-and-lowdb/node_modules/collect.js/docs/api/countBy.md

countBy()

The countBy method counts the occurences of values in the collection. By default, the method counts the occurrences of every element:

const collection = collect([1, 2, 2, 2, 3]);

const counted = collection.countBy();

counted.all();

// {
//   1: 1,
//   2: 3,
//   3: 1,
// }

However, you pass a callback to the countBy method to count all items by a custom value:

const collection = collect([
  'mohamed.salah@gmail.com',
  'sadio.mane@yahoo.com',
  'roberto.firmino@gmail.com',
]);

const counted = collection.countBy(email => email.split('@')[1]);

counted.all();

// {
//   'gmail.com': 2,
//   'yahoo.com': 1,
// }

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