_.each = _.forEach = function(obj, iteratee, context) {
iteratee = optimizeCb(iteratee, context);
var i, length;
if (isArrayLike(obj)) {
 for (i = 0, length = obj.length; i < length; i++) {
  iteratee(obj[i], i, obj);
 }
} else {
 var keys = _.keys(obj);
 for (i = 0, length = keys.length; i < length; i++) {
  iteratee(obj[keys[i]], keys[i], obj);
 }
}
// 链式调用
return obj;
 }; 

var keys = !isArrayLike(obj) && _.keys(obj),
    length = (keys || obj).length,
    results = Array(length);
  for (var index = 0; index < length; index++) {
   var currentKey = keys ? keys[index] : index;
   results[index] = iteratee(obj[currentKey], currentKey, obj);
  }
  return results;
  // 合理使用&&、||、?:可以大大减少代码量 

// js的最大精确整数
 var MAX_ARRAY_INDEX = Math.pow(2, 53) - 1;
 var isArrayLike = function(collection) {
var length = collection != null && collection.length;
return typeof length == 'number' && length >= 0 && length <= MAX_ARRAY_INDEX;
 }; // 如果集合有Length属性且为数字并且大于0小于最大的精确整数，则判定是类数组

 _.find = _.detect = function(obj, predicate, context) {
  var key;
  if (isArrayLike(obj)) {
   key = _.findIndex(obj, predicate, context);
  } else {
   key = _.findKey(obj, predicate, context);
  }
  if (key !== void 0 && key !== -1) return obj[key];
 };
function createIndexFinder(dir) {
  return function(array, predicate, context) {
   predicate = cb(predicate, context);
   var length = array != null && array.length;
   // 如果dir为1,index为0，index+=1，index正序循环
   // 如果dir 为-1，index为length-1,index += -1反序循环
   // 判断循环条件则用了index >= 0 && index < length方法兼顾两种循环方式
   var index = dir > 0 ? 0 : length - 1;
   for (; index >= 0 && index < length; index += dir) {
    if (predicate(array[index], index, array)) return index;
   }
   return -1;
  };
 }
 _.findIndex = createIndexFinder(1)；
 _.findLastIndex = createIndexFinder(-1)； 

_.max = function(obj, iteratee, context) {
var result = -Infinity, lastComputed = -Infinity,
  value, computed;
if (iteratee == null && obj != null) {
 obj = isArrayLike(obj) ? obj : _.values(obj);
 for (var i = 0, length = obj.length; i < length; i++) {
  value = obj[i];
  if (value > result) {
   result = value;
  }
 }
} else {
 iteratee = cb(iteratee, context);
 _.each(obj, function(value, index, list) {
  computed = iteratee(value, index, list);
  if (computed > lastComputed || computed === -Infinity && result === -Infinity) {
   result = value;
   lastComputed = computed;
  }
 });
}
return result;
 }; 

 _.toArray = function(obj) {
    if (!obj) return [];
    // 如果是数组，采用了Array.prototype.slice.call(this,obj)这种方法
    if (_.isArray(obj)) return slice.call(obj);
    // 类数组对象，这里没有采用Slice方法，而是利用.map对集合进行迭代，从而返回一个数组。 _.identity该方法传入的值和返回的值相等。（主要用于迭代）
    if (isArrayLike(obj)) return _.map(obj, _.identity);
    // 普通对象，则返回由属性值组成的数组。
    return _.values(obj);
   };