if isset statement with javascript?

Question

În php I would use this to see if a variable is set and then use that value, otherwise make it a zero:

 $pic_action = isset($_POST['pic_action']) ? $_POST['pic_action'] : 0;

But what is the equivalent in javascript?

I want to check if an element exists in the document, and then if it does, add it to the variable, otherwise add some other value to it, here is what I have so far:

 var areaOption = document.getElementById("element");

Thanks

Answer 1

var areaOption = document.getElementById("element") || some_other_variable;

Answer 2

isset() checks whether the variable is defined and not null. Javascript scoping rules being different from PHP, checking whether a variable is defined is harder. You can, on the other hand, determine if it is null:

if (areaOption === null) areaOption = ... ; 

Then, there's the classic Javascript idiom using the lazy evaluation of ||:

areaOption = areaOption || ... ;

The latter does not check for the variable being null, merely if it's equivalent to false, which means the right-hand part will be evaluated even if the value is defined (and is 0 or ''), which may be unsafe if you rely on the types being valid.

Answer 3

The more verbose approach:

var c = document.getElementById("a") ? document.getElementById("a") : "b";

Or the shorter version:

var c = document.getElementById("a") || "b";

If you want to access a property, say innerHTML, on a given element, if the element exists; the first example can be re-written, like so:

var c = document.getElementById("a") ?
 document.getElementById("a").innerHTML /*element exists: return innerHTML*/ :
 "" /*element does not exist: return nothing*/;

... Without the comments:

var c = document.getElementById("a") ?
 document.getElementById("a").innerHTML :
 "";

Answer 4

document.getElementById("element") will return null if the element doesn't exist -- and something not-null if it exists.

Which means you should be able to do something like this :

var areaOption = document.getElementById("element");
if (areaOption) {
 // element exists
} else {
 // element doesn't exist
}

(You could also compare areaOption with null, instead of just checking if it's non-falsy)

• cllpse: This is hardly replicating the isset() function in PHP.
• Etan: or use typeof (variable) == 'undefined' to check.

Answer 5

I believe what you're trying to accomplish is something like this:

var dummy = null;
var areaOption = (dummy = document.getElementById("element")) ? dummy.value : 0;