Possible Duplicate:
How can I check whether a variable is defined in JavaScript?
Say we have a piece of code like this. How would one be able to check whether the variable does exist or not in the case its value might be undefined?
var a = { foo: 'bar' }
a['foo'] = undefined;
// now a['foo'] returns undefined, as it does exist and contains undefined as its value
delete a['foo']
// now a['foo'] still returns undefined, but it doesn't exist
Thanks.
For this you use the in operator.
var a = {'foo': undefined};
'foo' in a // returns true
delete a.foo;
'foo' in a // returns false
alternatively you can use Object.prototype.hasOwnProperty
var a = {'foo': undefined};
a.hasOwnProperty('foo') // returns true
delete a.foo;
a.hasOwnProperty('foo') // returns false
Setting a['foo'] = undefined is equivalent to delete a['foo']
If you print the value of a['foo'] after setting it to undefined, you will see that it returns and empty variable/struct.
If you were trying to set a['foo'] to a string with "undefined", you could have used the "typeof" function to check if it is indeed undefined or if it is a string.
EDIT:
You can check if it exists or not using 'foo' in a
i.e.
'foo' in a // returns true after setting it to undefined 'foo' in a //returns false after deleting it.
You can check if the variable exists by passing it to an if-statement:
if (a['foo'])
alert("a['foo'] does exist.");
