Compatibility problems of for ... in Back

As we known, for ... in will only loop through all the enumerable attributes. Objects created by the inner constructors of Array and Object will inherit all the non-enumerable attributes like indexOf() from String, or toString() from Object. for ... in will iterate to loop through all the enumerable attributes of objects and attributes inherited from their constructors. (include overridden built-in attributes)

var obj = {
    name: 'hanzichi',
    age: 30

for (var k in obj) {
    console.log(k, obj[k]);

 * output:
 *  => name hanzichi
 *  => 30

However, there is a compatibility problem in IE8 (Apparently, IE8 has treated toString as a non-enumerable attribute, even if it's overridden):

var obj = { toString: 'hanzichi' };

for (var k in obj) {

 * output:
 *  => toString (Chrome)
 *  => nothing hanppen in IE*

Therefore, in underscore.js, there is a method named hasEnumBug to get bug from IE8:

 * Keys in IE < 9 that won't be iterated by `for key in ...` and thus missed.
 * And the but can be judged by checking whether `toString` can be overriden */
var hasEnumBug = !{ toString: null }.propertyIsEnumerable('toString');

Besides, underscore.js has listed out all the familiar situation in the set nonEnumerableProps.

/** the keys which can not be iterated with for ... in */
var nonEnumerableProps = ['valueOf', 'isPrototypeOf', 'toString', 'propertyIsEnumerable', 'hasOwnProperty', 'toLocaleString'];

If some has been ignored, it will also provide a method used for collecting, named collectNonEnumProps.

function collectNonEnumProps(obj, keys) {
    var nonEnumIdx = nonEnumerableProps.length;
    var constructor = obj.constructor;

    /** check whether `proto` is inherited */
    var proto = (_.isFunction(constructor) && constructor.prototype) || ObjProto;

     * Constructor is a special case.
     * if the object has `constructor` but the key is not included in the array `keys`, then save it in it
    var prop = 'constructor';
    if (_.has(obj, prop) && !_.contains(keys, prop)) keys.push(prop);

    /** check the `nonEnumerableProps` array */
    while (nonEnumIdx--) {
        prop = nonEnumerableProps[nonEnumIdx];
         * obj[prop] !== proto[prop] is used to check whether the property is from the inherited prototype
         * then check whether the array `keys` has contained non-enumerable properties
        if (prop in obj && obj[prop] !== proto[prop] && !_.contains(keys, prop)) {

After overriding all the attributes, keys can be gotten under IE 9. However, there are still a doubt why handling constructor specially.

Empty Comments
Sign in GitHub

As the plugin is integrated with a code management system like GitLab or GitHub, you may have to auth with your account before leaving comments around this article.

Notice: This plugin has used Cookie to store your token with an expiration.