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

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?

Why not `hasOwnProperty`? (Excepting aesthetic reasons.) The difference between that an `in` is that `in` will traverse the [[prototype]] as well. — , May 30 '12 at 19:36
`mmap.has(key)` is latest ECMAScript 6 way of checking the existance of a key in a map. [Refer to this](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/has) for complete details. — Diablo, May 30 '17 at 08:59

Kendall Frey · Accepted Answer · 2012-05-30T19:46:43.410

59

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.

edited May 30 '12 at 19:46

answered May 30 '12 at 19:31

Kendall Frey

43,130
20
110
148

score 10 · Answer 2 · answered May 30 '12 at 19:32

10

You can use:

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

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

answered May 30 '12 at 19:32

Mike Christensen

88,082
50
208
326

1

However, if the property was set to undefined, it still exists, but this will return false. – Kendall Frey May 30 '12 at 19:34
2

`{x: undefined}.x === undefined` is true – May 30 '12 at 19:36
2

Valid point, not to mention if some idiot redefines *undefined* – Mike Christensen May 30 '12 at 19:40

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

2 Answers2