countBy.js

var createAggregator = require('../internal/createAggregator');

/** Used for native method references. */
var objectProto = Object.prototype;

/** Used to check objects for own properties. */
var hasOwnProperty = objectProto.hasOwnProperty;

/**
 * Creates an object composed of keys generated from the results of running
 * each element of `collection` through `iteratee`. The corresponding value
 * of each key is the number of times the key was returned by `iteratee`.
 * The `iteratee` is bound to `thisArg` and invoked with three arguments:
 * (value, index|key, collection).
 *
 * If a property name is provided for `iteratee` the created `_.property`
 * style callback returns the property value of the given element.
 *
 * If a value is also provided for `thisArg` the created `_.matchesProperty`
 * style callback returns `true` for elements that have a matching property
 * value, else `false`.
 *
 * If an object is provided for `iteratee` the created `_.matches` style
 * callback returns `true` for elements that have the properties of the given
 * object, else `false`.
 *
 * @static
 * @memberOf _
 * @category Collection
 * @param {Array|Object|string} collection The collection to iterate over.
 * @param {Function|Object|string} [iteratee=_.identity] The function invoked
 *  per iteration.
 * @param {*} [thisArg] The `this` binding of `iteratee`.
 * @returns {Object} Returns the composed aggregate object.
 * @example
 *
 * _.countBy([4.3, 6.1, 6.4], function(n) {
 *   return Math.floor(n);
 * });
 * // => { '4': 1, '6': 2 }
 *
 * _.countBy([4.3, 6.1, 6.4], function(n) {
 *   return this.floor(n);
 * }, Math);
 * // => { '4': 1, '6': 2 }
 *
 * _.countBy(['one', 'two', 'three'], 'length');
 * // => { '3': 2, '5': 1 }
 */
var countBy = createAggregator(function(result, value, key) {
  hasOwnProperty.call(result, key) ? ++result[key] : (result[key] = 1);
});

module.exports = countBy;