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);      }    }};