underscore的reduce和reduceRight为什么不使用native function ?

在读underscore的源码，有一个疑问，为什么不优先选择原生的reduce和reduceRight

源码中，比如 _.keys，都是先探测是否支持Object.keys的。

 // Create a reducing function iterating left or right.  // ? why dont use native reduce  var createReduce = function(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);    };  };// **Reduce** builds up a single result from a list of values, aka `inject`,// or `foldl`._.reduce = _.foldl = _.inject = createReduce(1);// The right-associative version of reduce, also known as `foldr`._.reduceRight = _.foldr = createReduce(-1);