对象是空的吗？

Question

检查对象是否为空的最快方法是什么？

有没有比这更快更好的方法：

function count_obj(obj){
    var i = 0;
    for(var key in obj){
        ++i;
    }

    return i;
}

Answer 1

我假设用 空 表示“没有任何属性”。

// Speed up calls to hasOwnProperty
var hasOwnProperty = Object.prototype.hasOwnProperty;

function isEmpty(obj) {

    // null and undefined are "empty"
    if (obj == null) return true;

    // Assume if it has a length property with a non-zero value
    // that that property is correct.
    if (obj.length > 0)    return false;
    if (obj.length === 0)  return true;

    // If it isn't an object at this point
    // it is empty, but it can't be anything *but* empty
    // Is it empty?  Depends on your application.
    if (typeof obj !== "object") return true;

    // Otherwise, does it have any properties of its own?
    // Note that this doesn't handle
    // toString and valueOf enumeration bugs in IE < 9
    for (var key in obj) {
        if (hasOwnProperty.call(obj, key)) return false;
    }

    return true;
}

例子：

isEmpty(""), // true
isEmpty(33), // true (arguably could be a TypeError)
isEmpty([]), // true
isEmpty({}), // true
isEmpty({length: 0, custom_property: []}), // true

isEmpty("Hello"), // false
isEmpty([1,2,3]), // false
isEmpty({test: 1}), // false
isEmpty({length: 3, custom_property: [1,2,3]}) // false

如果只需要处理ECMAScript5浏览器，则可以使用Object.getOwnPropertyNames而不是hasOwnProperty循环：

if (Object.getOwnPropertyNames(obj).length > 0) return false;

这将确保即使对象仅具有不可枚举的属性，isEmpty也仍将为您提供正确的结果。