How can I tell if an object is blank?

If I set an object using:

var a = {};

or

var a = new Object();

How can I test whether or not it has anything inside it if I don't know specifically what to test for.开发者_如何转开发

(something like a == {}, but this does not evaluate correctly)

You could use this simple function:

function isEmpty(object) {
    for(var property in object) {
        if(object.hasOwnProperty(property))
            return false;
    }

    return true;
}

But if you use jQuery: http://api.jquery.com/jQuery.isEmptyObject/

This can seem an innocent question, a simple solution as the one @SiteSafeNL provides could seem to work, but there is an implementation bugs that is not taken in account, the IE {DontEnum} Bug.

If you target IE8 and below you will need to implement something like this:

var isEmpty = (function () {
    var hasOwn = Object.prototype.hasOwnProperty,
        hasDontEnumBug = !{toString:0}.propertyIsEnumerable("toString"),
        dontEnums = ['toString', 'toLocaleString',
            'valueOf', 'hasOwnProperty',
            'isPrototypeOf', 'propertyIsEnumerable',
            'constructor'];


    return function (o) {
        for (var p in o) {
            if (hasOwn.call(o, p))
                return false
        }

        if (hasDontEnumBug) { // IE8 and below
            var i = dontEnums.length;
            while(i--) {
                if (hasOwn.call(o, dontEnums[i]))
                    return false;
            }
        }

        return true;
    };
})();

Now in modern browsers that implement ECMAScript 5 as @Alnitak mentions you could use Object.keys, but in ES5 now we have the power of defining non-enumerable properties, these properties won't appear on the for-in loop, if you need to detect them, you have to use an alternate method, for example, using the above function, or Object.keys:

var obj = Object.create(null, { foo: { value: 1, /* enumerable:false... */} });
isEmpty(obj); // true
Object.keys(obj).length; // 0

// But the property exists:
obj.hasOwnProperty('foo'); // true
obj.foo; // 1

We need another approach if you want to catch non-enumerable own properties, that's why the Object.getOwnPropertyNames method exist:

function isEmpty(obj) {
  return Object.getOwnPropertyNames(obj).length == 0;
}

In ECMAScript 5 you can use:

if (Object.keys(a).length === 0) {
    ...
}

NB: this will only find enumerable properties.