Function iterator

public/javascripts/underscore.js:181–187

(obj, iteratee, memo, keys, index, length)

    // Optimized iterator function as using arguments.length
    // in the main function will deoptimize the, see #1991.
    function iterator(obj, iteratee, memo, keys, index, length) {
      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) {
      iteratee = optimizeCb(iteratee, context, 4);