jashkenas/underscore

import isArrayLike from './_isArrayLike.js';
import keys from './keys.js';
import optimizeCb from './_optimizeCb.js';

// Internal helper to create a reducing function, iterating left or right.
export default function createReduce(dir) {
  // Wrap code that reassigns argument variables in a separate function than
  // the one that accesses `arguments.length` to avoid a perf hit. (#1991)
  var reducer = function(obj, iteratee, memo, initial) {
    var _keys = !isArrayLike(obj) && keys(obj),
        length = (_keys || obj).length,
        index = dir > 0 ? 0 : length - 1;
    if (!initial) {
      memo = obj[_keys ? _keys[index] : index];
      index += dir;
    }
    for (; index >= 0 && index < length; index += dir) {
      var currentKey = _keys ? _keys[index] : index;
      memo = iteratee(memo, obj[currentKey], currentKey, obj);
    }
    return memo;
  };

  return function(obj, iteratee, memo, context) {
    var initial = arguments.length >= 3;
    return reducer(obj, optimizeCb(iteratee, context, 4), memo, initial);
  };
}