How to check for undefined parameter in function calls

medini_1024169437

I myself typically check to an undefined, and thus default parameter in a cunfction call like this:

function c2(p) {
	return !p;
}

So I basically use the ! operator. Drawback is that !0 and !"" is also true.

So when I recently stumbled over this code, I thought I found a better solution:

function c4(p) {
	return (typeof p) === 'undefined';
}

My first thought was: why so complicated? I searched the web and the pro of this solution is that the variable under test doe snot evene need to be defined. Great. However, I stll was not convinced, especially as Rhino sometime behaves differently than other engines. So I wrote quickly a small test script like this:

// $EXPERIMENTAL$ $STRICT_MODE$ $WARNING_AS_ERROR$ 
function c1(p) {
	return p === undefined;
}
function c2(p) {
	return !p;
}
function c3(p) {
	return p == undefined;
}
function c4(p) {
	return (typeof p) === 'undefined';
}

console.log("c1");
console.log(c1());
console.log(c1(undefined));
console.log(c1(null));

console.log("\nc2");
console.log(c2());
console.log(c2(undefined));
console.log(c2(null));

console.log("\nc3");
console.log(c3());
console.log(c3(undefined));
console.log(c3(null));

console.log("\nc4");
console.log(c4());
console.log(c4(undefined));
console.log(c4(null));

And I was a bit surprised about the outcome: 

c1
true
true
false

c2
true
true
true

c3
true
true
true

c4
true
true
false 

So only C2 and C3 bring the desired outcome but not C4?

I have not further investigated yet but wanted to post already.

Note that I will be not reading this board for the a couple of weeks, basically April and May. Will be back in June.

Happy Scripting

Jan  

medini_1063548292

Hi Jan,

I have same experience and concluded that the only reliable way is using  ==undefined (C3 in your example) and that covers null & undefined and 'undefined' at the same time.

Note; that (C1 !p1;) will work for false,0,null -> I do not like to use that if I am really looking towards only a case without a definition or null, and the 0 as number is a valid value that should not trigger the check.

The C4 does not work because the null is considered as a JavaObject by Rhino == typeof null  =>'object'

PS: All you mention works without rising an error until you are in the true strict mode.

one has to be careful if testing for a global variable because a simple gVar1 == undefined will trigger an JavaScript exception.

If one wants to check if some variable (global, local) is existing Inside of a function there are only couple of ways you can use without rising an exception (try catch can cover this).

Testing the global variable presence within a function working also with "use strict" might be tricky, I need to also play around that in the strict mode especially with the fact that Rhino might implement it a bit different. implementing it.

Marian