CountBy

CountBy is a utility function found in libraries such as lodash. It creates an object composed of keys generated by running each element of a collection through an iteratee, with the value for each key being the number of elements that produced that key.

The typical signature accepts a collection (an array, an array-like object, or an object) and an iteratee. If the iteratee is omitted, the identity of the element is used. The iteratee can be a function, a property name, or another expression that yields a key. For function iteratees, each element is passed to the function (value, index|key, collection), and the resulting key is converted to a string to serve as a property name in the output object.

The return value is an object whose enumerable properties represent the computed keys and whose values are the counts of elements that produced each key. In common implementations, missing or undefined keys can appear as the string 'undefined' in the result.

Example: countBy([6.1, 4.2, 6.3], Math.floor) yields { '4': 1, '6': 2 }. Another example: countBy(['apple', 'banana', 'apricot'], x => x[0]) yields { a: 2, b: 1 }.

Compared with groupBy, which groups elements into arrays by key, countBy returns only the frequencies. It is a common utility in lodash and similar libraries and can often be emulated with a reduce in languages without a built-in function. Time complexity is O(n) and space complexity is O(k), where k is the number of distinct keys.