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countBy javascript Code Snippet in 2023

arrayobjectintermediate

Last Updated: 28 January 2023

Groups the elements of an array based on the given function and returns the count of elements in each group.

  • Use Array.prototype.map() to map the values of an array to a function or property name.
  • Use Array.prototype.reduce() to create an object, where the keys are produced from the mapped results.
const countBy = (arr, fn) => arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => { acc[val] = (acc[val] || 0) + 1; return acc; }, {});
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}

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