javascript测试对象属性

Question

I have a string like this:

var TheDate = "6.14.2012";

and I have an object MyObject that has properties that may match that string. However, when I write this:

if (MyObject[TheDate] === null || MyObject[TheDate] === 'undefined') { .... }

the conditions never seem to evaluate to true even though I know the property doesn't exist.

What am I missing??

Thanks.

Answer 1

Existence of Own Property

If you want to check for the existence of a property use .hasownProperty() ...

if (MyObject.hasOwnProperty(TheDate)) {

---

Existence of Own Or Inherited Property

If the property may be inherited, to test for existence use the in operator...

if (TheDate in MyObject) {

---

null or undefined value test for Own Or Inherited Property

If it's not existence, but rather a value test, and you want to test for null or undefined , then use this...

if (MyObject[TheDate] == null) {

This will check for both null and undefined .

Answer 2

The problem is with the quotes around undefined ;)

Change

if (MyObject[TheDate] === null || MyObject[TheDate] === 'undefined') { .... }

to

if (MyObject[TheDate] === null || MyObject[TheDate] === undefined) { .... }

It should work :)

Answer 3

If you want to use the string "undefined" rather than the undefined object, the way to do this is to check the "typeof", which returns a string. In this case, if you're looking to test if it's set to "undefined":

if ( MyObject[TheDate] === null || typeof MyObject[TheDate] === 'undefined' ) { .... }

Answer 4

Try the following

if ( (MyObject[TheDate] === null) || (typeof MyObject[TheDate] === 'undefined') { 
 .... 
}

You were checking whether your property contains the String 'undefined' and not whether its value is currently undefined .