reduceBy.js•2.1 kB
var _curryN = require('./internal/_curryN');
var _has = require('./internal/_has');
var _reduce = require('./internal/_reduce');
/**
* Groups the elements of the list according to the result of calling
* the String-returning function `keyFn` on each element and reduces the elements
* of each group to a single value via the reducer function `valueFn`.
*
* This function is basically a more general `groupBy` function.
*
* @func
* @memberOf R
* @since v0.20.0
* @category List
* @sig ((a, b) -> a) -> a -> (b -> String) -> [b] -> {String: a}
* @param {Function} valueFn The function that reduces the elements of each group to a single
* value. Receives two values, accumulator for a particular group and the current element.
* @param {*} acc The (initial) accumulator value for each group.
* @param {Function} keyFn The function that maps the list's element into a key.
* @param {Array} list The array to group.
* @return {Object} An object with the output of `keyFn` for keys, mapped to the output of
* `valueFn` for elements which produced that key when passed to `keyFn`.
* @see R.groupBy, R.reduce
* @example
*
* var reduceToNamesBy = R.reduceBy((acc, student) => acc.concat(student.name), []);
* var namesByGrade = reduceToNamesBy(function(student) {
* var score = student.score;
* return score < 65 ? 'F' :
* score < 70 ? 'D' :
* score < 80 ? 'C' :
* score < 90 ? 'B' : 'A';
* });
* var students = [{name: 'Lucy', score: 92},
* {name: 'Drew', score: 85},
* // ...
* {name: 'Bart', score: 62}];
* namesByGrade(students);
* // {
* // 'A': ['Lucy'],
* // 'B': ['Drew']
* // // ...,
* // 'F': ['Bart']
* // }
*/
module.exports = _curryN(4, [], function reduceBy(valueFn, valueAcc, keyFn, list) {
return _reduce(function(acc, elt) {
var key = keyFn(elt);
acc[key] = valueFn(_has(key, acc) ? acc[key] : valueAcc, elt);
return acc;
}, {}, list);
});