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x/ramda/reduceBy.js

:ram: Practical functional Javascript
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import _curryN from './internal/_curryN';import _dispatchable from './internal/_dispatchable';import _has from './internal/_has';import _reduce from './internal/_reduce';import _xreduceBy from './internal/_xreduceBy';

/** * 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`](#groupBy) function. * * Acts as a transducer if a transformer is given in list position. * * @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 * * const groupNames = (acc, {name}) => acc.concat(name) * const toGrade = ({score}) => * score < 65 ? 'F' : * score < 70 ? 'D' : * score < 80 ? 'C' : * score < 90 ? 'B' : 'A' * * var students = [ * {name: 'Abby', score: 83}, * {name: 'Bart', score: 62}, * {name: 'Curt', score: 88}, * {name: 'Dora', score: 92}, * ] * * reduceBy(groupNames, [], toGrade, students) * //=> {"A": ["Dora"], "B": ["Abby", "Curt"], "F": ["Bart"]} */var reduceBy = _curryN(4, [], _dispatchable([], _xreduceBy, 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); }));export default reduceBy;