title: countBy tags: array,object,intermediate

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.

export const countBy = <T = any>(arr: T[], fn: Func<T> | string) => {
  const mapper = typeof fn === "function" ? fn : (val: any) => val[fn];
  return arr.reduce((acc, val) => {
    const value = mapper(val);
    acc[value] = (acc[value] || 0) + 1;
    return acc;
  }, {} as any);
};

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

countBy([{ name: "Deepak" }, { name: "Deepak2" }, { name: "Deepak" }], "name"); // { Deepak: 2, Deepak2: 1 }
countBy(
  [{ name: "Deepak" }, { name: "Deepak2" }, { name: "Deepak" }],
  (user: any) => user.name
); // { Deepak: 2, Deepak2: 1 }