Function reduce

Reduces collection to a value which is the accumulated result of running each element in collection thru iteratee, where each successive invocation is supplied the return value of the previous. If accumulator is not given, the first element of collection is used as the initial value. The iteratee is invoked with four arguments: (accumulator, value, index|key, collection).

Many lodash methods are guarded to work as iteratees for methods like reduce, reduceRight, and transform.

The guarded methods are: assign, defaults, defaultsDeep, includes, merge, orderBy, and sortBy

reduce([1, 2], (sum, n) => sum + n, 0)
// => 3

reduce({ 'a': 1, 'b': 2, 'c': 1 }, (result, value, key) => {
  (result[value] || (result[value] = [])).push(key)
  return result
}, {})
// => { '1': ['a', 'c'], '2': ['b'] } (iteration order is not guaranteed)
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