如何检查 Javascript 对象是否具有给定键的值？

How to check whether a Javascript object has a value for a given key?

提问by Jér?me Verstrynge

Possible Duplicate:
How do I check to see if an object has an attribute in Javascript?

可能的重复：
如何检查对象是否在 Javascript 中具有属性？

I have a Javascript object defined as following:

我有一个 Javascript 对象定义如下：

var mmap = new Object();

mmap['Q'] = 1;
mmap['Z'] = 0;
mmap['L'] = 7;
...

How to check whether this map has a value for a given key (for example 'X')? Does .hasOwnProperty()get into play?

如何检查此映射是否具有给定键的值（例如“X”）？是否.hasOwnProperty()进入游戏？

回答by Kendall Frey

if ('X' in mmap)
{
    // ...
}

Hereis an example on JSFiddle.

这是一个关于 JSFiddle 的例子。

hasOwnPropertyis also valid, but using inis much more painless. The only difference is that inreturns prototype properties, whereas hasOwnPropertydoes not.

hasOwnProperty也是有效的，但使用in起来更轻松。唯一的区别是in返回原型属性，而hasOwnProperty不会。

回答by Mike Christensen

You can use:

您可以使用：

(mmap['X'] === undefined)

Fiddle: http://jsfiddle.net/eDTrY/

小提琴：http: //jsfiddle.net/eDTrY/