ie9下对象for..in..的bug

Underscore的源码中有这样几行

  // Keys in IE < 9 that won't be iterated by `for key in ...` and thus missed.
  var hasEnumBug = !{toString: null}.propertyIsEnumerable('toString');
  var nonEnumerableProps = ['valueOf', 'isPrototypeOf', 'toString',
                      'propertyIsEnumerable', 'hasOwnProperty', 'toLocaleString'];

ie9中，nonEnumerableProps数组中所包含的属性，不能通过for key in 这样的方式取得。

所以underscore在实现_.keys的时候，判断是否有这样的bug存在。

_.keys = function(obj) {
    if (!_.isObject(obj)) return [];
    if (nativeKeys) return nativeKeys(obj);
    var keys = [];
    for (var key in obj) if (_.has(obj, key)) keys.push(key);
    // Ahem, IE < 9.
    if (hasEnumBug) collectNonEnumProps(obj, keys);
    return keys;
};

在collectNonEnumProps中，将constructor和nonEnumerableProps全部添加到keys

var collectNonEnumProps = function(obj, keys) {
    var nonEnumIdx = nonEnumerableProps.length;
    var constructor = obj.constructor;
    var proto = _.isFunction(constructor) && constructor.prototype || ObjProto;

    // Constructor is a special case.
    var prop = 'constructor';
    if (_.has(obj, prop) && !_.contains(keys, prop)) keys.push(prop);

    while (nonEnumIdx--) {
      prop = nonEnumerableProps[nonEnumIdx];
      if (prop in obj && obj[prop] !== proto[prop] && !_.contains(keys, prop)) {
        keys.push(prop);
      }
    }
};