countBy.js

import baseAssignValue from './.internal/baseAssignValue.js';
import reduce from './reduce.js';

/** Used to check objects for own properties. */
const hasOwnProperty = Object.prototype.hasOwnProperty;

/**
 * Creates an object composed of keys generated from the results of running
 * each element of `collection` thru `iteratee`. The corresponding value of
 * each key is the number of times the key was returned by `iteratee`. The
 * iteratee is invoked with one argument: (value).
 *
 * @since 0.5.0
 * @category Collection
 * @param {Array|Object} collection The collection to iterate over.
 * @param {Function} iteratee The iteratee to transform keys.
 * @returns {Object} Returns the composed aggregate object.
 * @example
 *
 * countBy([6.1, 4.2, 6.3], Math.floor);
 * // => { '4': 1, '6': 2 }
 */
function countBy(collection, iteratee) {
  return reduce(collection, (result, value, key) => {
    if (hasOwnProperty.call(result, key)) {
      ++result[key];
    } else {
      baseAssignValue(result, key, 1);
    }
    return result;
  }, {});
}

export default countBy;