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

Question

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

I have a Javascript object defined as following:

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?

Answer 1

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

Here is an example on JSFiddle.

hasOwnProperty is also valid, but using in is much more painless. The only difference is that in returns prototype properties, whereas hasOwnProperty does not.

Answer 2

You can use:

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

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