Can not quite understand the reasoning for this. In the following code the localStorage of an item is alerted as undefined, but if I use an if(x==undefined) syntax it does not work. Can somebody explain what is the problem. Thank you.
alert(localStorage["x"]);
if(localStorage["x"]=="undefined"){alert("y");}
The top line alerts undefined
The bottom line does not alert y for me.
It doesn't contain the string "undefined", it contains a value of the type undefined:
if (localStorage["x"] == undefined) { alert("y"); }
The value undefined is possible to change in older browsers, so good practice is to check the type instead:
if (typeof localStorage["x"] == 'undefined') { alert("y"); }
Try:
if(typeof( localStorage["x"]) == 'undefined'){alert("y");}
OR
if( localStorage["x"] == undefined){alert("y");}
OR
if( !localStorage["x"] ){alert("y");}
The two ways of checking for something being undefined are:
typeof foo === "undefined"
and
foo === undefined
In the first case, it will be true if foo was never defined or the value of foo is undefined.
In the second case, it will only be true if foo was defined (otherwise it'll break) and its value is undefined.
Checking its value against the string "undefined" is not the same at all!
UPDATE:
When I said that if you try to perform an operation on an object literal's property that isn't defined, I guess I meant if it's undefined at all, and what I meant was something more like this:
obj["x"].toLowerCase()
// or
obj["x"]["y"]
where you are attempting to access/operate on something that is originally undefined. In this case, simply comparing in an if statement should be fine, because of the way object literals report the value...but is very different with normal Javascript variables.
With object literals, if a key (say "x") is not defined, then
obj["x"]
returns a value of undefined, so both the typeof and basic === undefined checks will work and be true.
The whole difference of not being defined or having a value of undefined is different with normal variables.
If you had:
var a;
// or
var a = undefined;
then both the typeof and basic === undefined checks I provided earlier would work and be true. But if you never even declared a, then only the typeof check would work and be true. The === undefined check would break.
Take a look at: http://jsfiddle.net/7npJx/
If you notice in the console, it says b is not defined and breaks the if statement.
Since you're basically looking at an object literal with localStorage, the way to distinguish whether an item is not defined or has a value of undefined is to use in first. So, you could use:
if (!("x" in localStorage)) {
to check if "x" is not a defined property at all, and:
else if (localStorage["x"] === undefined) {
to then check if it is defined but has a value of undefined. Then, using:
else {
would signify that localStorage["x"] is both defined and does not have the value undefined.
In your code though, it's okay to use the typeof or in checks (based on what you want to know) because of the way object literals report properties that aren't defined. Using the basic === undefined is also okay, but as Guffa pointed out, it's possible for the actual value of undefined to be overwritten and then wouldn't work in this comparison. When it comes to normal Javascript variables, typeof and === undefined checks aren't the same.
I've been working on a javascript library, and I have a lot of redundant checks like this:
if(typeof foo !== "undefined" && foo !== null)
So, I wanted to create a function that will be a shortcut to this unwieldy check. So I came up with this:
function isset(a)
{
return (typeof a !== "undefined" && a !== null) ? true : false;
}
But, since the value could be undefined, and it attempts to use a possibly undefined variable, it turns out to be useless.
Is there a way to accomplish this without have to extend a native prototype?
It really depends on what you mean by undefined.
1. You mean the variable does not exist.
In this case, what you want is not possible. typeof is an operator and therefore has magic behavior you just can't emulate using the language. If you try to pass a variable that doesn't exist to your function, it will throw a ReferenceError.
(See below for a workaround.)
2. You mean the variable has the value undefined, but does exist.
In this case, your function will do the trick -- though it could be simplified to the following:
function isset(variable) {
return variable != null;
}
This function will return false if the variable is either undefined or null. It takes advantage of the fact that undefined == null in JavaScript. Of course, with such a short function, one could argue that the function isn't needed at all.
Recall that a variable that is declared has a value -- undefined -- by default.
The name of your function suggests you mean case #1. I don't know what sort of library you are writing, but I can't imagine a case in a library where you would need to check if a variable exists, though I can definitely think of many possibilities for case #2.
If case #1 is necessary, remember that you can re-declare variables without changing their value:
a = 1; // pretend this was set somewhere higher up in the code
var a; // this does not change the value of `a`
If you re-declare variables before you use isset, you could avoid the ReferenceError problem. You won't be able to tell if the code has already declared the variable, though; you will only be able to tell if they have not assigned it to some other value.
You can check for null and undefined at the same time by using != instead.
// checks for both null and undefined
if( foo != null ) { ...
...so no need to use a function to shorten it.
null and undefined also evaluates to false in an if statement. So the following statement also works:
if(!foo)
...
This should cut it down significantly.
I do not understand why people keep promoting if (var) or if (!var). Both fail in my browsers. Meantime if (obj.prop) passes. This means that we should use if (this.someVar) or if (window.someVar) instead of if (varbl). Ok?
Which method of checking if a variable has been initialized is better/correct?
(Assuming the variable could hold anything (string, int, object, function, etc.))
if (elem) { // or !elem
or
if (typeof elem !== 'undefined') {
or
if (elem != null) {
You want the typeof operator. Specifically:
if (typeof variable !== 'undefined') {
// the variable is defined
}
The typeof operator will check if the variable is really undefined.
if (typeof variable === 'undefined') {
// variable is undefined
}
The typeof operator, unlike the other operators, doesn't throw a ReferenceError exception when used with an undeclared variable.
However, do note that typeof null will return "object". We have to be careful to avoid the mistake of initializing a variable to null. To be safe, this is what we could use instead:
if (typeof variable === 'undefined' || variable === null) {
// variable is undefined or null
}
For more info on using strict comparison === instead of simple equality ==, see:Which equals operator (== vs ===) should be used in JavaScript comparisons?
In many cases, using:
if (elem) { // or !elem
will do the job for you!... this will check these below cases:
undefined: if the value is not defined and it's undefined
null: if it's null, for example, if a DOM element not exists...
empty string: ''
0: number zero
NaN: not a number
false
So it will cover off kind of all cases, but there are always weird cases which we'd like to cover as well, for example, a string with spaces, like this ' ' one, this will be defined in javascript as it has spaces inside string... for example in this case you add one more check using trim(), like:
if(elem) {
if(typeof elem === 'string' && elem.trim()) {
///
Also, these checks are for values only, as objects and arrays work differently in Javascript, empty array [] and empty object {} are always true.
I create the image below to show a quick brief of the answer:
In JavaScript, a variable can be defined, but hold the value undefined, so the most common answer is not technically correct, and instead performs the following:
if (typeof v === "undefined") {
// no variable "v" is defined in the current scope
// *or* some variable v exists and has been assigned the value undefined
} else {
// some variable (global or local) "v" is defined in the current scope
// *and* it contains a value other than undefined
}
That may suffice for your purposes. The following test has simpler semantics, which makes it easier to precisely describe your code's behavior and understand it yourself (if you care about such things):
if ("v" in window) {
// global variable v is defined
} else {
// global variable v is not defined
}
This, of course, assumes you are running in a browser (where window is a name for the global object). But if you're mucking around with globals like this you're probably in a browser. Subjectively, using 'name' in window is stylistically consistent with using window.name to refer to globals. Accessing globals as properties of window rather than as variables allows you to minimize the number of undeclared variables you reference in your code (for the benefit of linting), and avoids the possibility of your global being shadowed by a local variable. Also, if globals make your skin crawl you might feel more comfortable touching them only with this relatively long stick.
In the majority of cases you would use:
elem != null
Unlike a simple if (elem), it allows 0, false, NaN and '', but rejects null or undefined, making it a good, general test for the presence of an argument, or property of an object.
The other checks are not incorrect either, they just have different uses:
if (elem): can be used if elem is guaranteed to be an object, or if false, 0, etc. are considered "default" values (hence equivalent to undefined or null).
typeof elem == 'undefined' can be used in cases where a specified null has a distinct meaning to an uninitialised variable or property.
This is the only check that won't throw an error if elem is not declared (i.e. no var statement, not a property of window, or not a function argument). This is, in my opinion, rather dangerous as it allows typos to slip by unnoticed. To avoid this, see the below method.
Also useful is a strict comparison against undefined:
if (elem === undefined) ...
However, because the global undefined can be overridden with another value, it is best to declare the variable undefined in the current scope before using it:
var undefined; // really undefined
if (elem === undefined) ...
Or:
(function (undefined) {
if (elem === undefined) ...
})();
A secondary advantage of this method is that JS minifiers can reduce the undefined variable to a single character, saving you a few bytes every time.
Check if window.hasOwnProperty("varname")
An alternative to the plethora of typeof answers;
Global variables declared with a var varname = value; statement in the global scope
can be accessed as properties of the window object.
As such, the hasOwnProperty() method, which
returns a boolean indicating whether the object has the specified property as its own property (as opposed to inheriting it)
can be used to determine whether
a var of "varname" has been declared globally i.e. is a property of the window.
// Globally established, therefore, properties of window
var foo = "whatever", // string
bar = false, // bool
baz; // undefined
// window.qux does not exist
console.log( [
window.hasOwnProperty( "foo" ), // true
window.hasOwnProperty( "bar" ), // true
window.hasOwnProperty( "baz" ), // true
window.hasOwnProperty( "qux" ) // false
] );
What's great about hasOwnProperty() is that in calling it, we don't use a variable that might as yet be undeclared - which of course is half the problem in the first place.
Although not always the perfect or ideal solution, in certain circumstances, it's just the job!
Notes
The above is true when using var to define a variable, as opposed to let which:
declares a block scope local variable, optionally initializing it to a value.
is unlike the var keyword, which defines a variable globally, or locally to an entire function regardless of block scope.
At the top level of programs and functions, let, unlike var, does not create a property on the global object.
For completeness: const constants are, by definition, not actually variable (although their content can be); more relevantly:
Global constants do not become properties of the window object, unlike var variables. An initializer for a constant is required; that is, you must specify its value in the same statement in which it's declared.
The value of a constant cannot change through reassignment, and it can't be redeclared.
The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned.
Since let variables or const constants are never properties of any object which has inherited the hasOwnProperty() method, it cannot be used to check for their existence.
Regarding the availability and use of hasOwnProperty():
Every object descended from Object inherits the hasOwnProperty() method. [...] unlike the in operator, this method does not check down the object's prototype chain.
How to check if a variable exists
This is a pretty bulletproof solution for testing if a variable exists and has been initialized :
var setOrNot = typeof variable !== typeof undefined;
It is most commonly used in combination with a ternary operator to set a default in case a certain variable has not been initialized :
var dark = typeof darkColor !== typeof undefined ? darkColor : "black";
Problems with encapsulation
Unfortunately, you cannot simply encapsulate your check in a function.
You might think of doing something like this :
function isset(variable) {
return typeof variable !== typeof undefined;
}
However, this will produce a reference error if you're calling eg. isset(foo) and variable foo has not been defined, because you cannot pass along a non-existing variable to a function :
Uncaught ReferenceError: foo is not defined
Testing whether function parameters are undefined
While our isset function cannot be used to test whether a variable exists or not (for reasons explained hereabove), it does allow us to test whether the parameters of a function are undefined :
var a = '5';
var test = function(x, y) {
console.log(isset(x));
console.log(isset(y));
};
test(a);
// OUTPUT :
// ------------
// TRUE
// FALSE
Even though no value for y is passed along to function test, our isset function works perfectly in this context, because y is known in function test as an undefined value.
Short way to test a variable is not declared (not undefined) is
if (typeof variable === "undefined") {
...
}
I found it useful for detecting script running outside a browser (not having declared window variable).
There is another short hand way to check this, when you perform simple assignments and related checks. Simply use Conditional (Ternary) Operator.
var values = typeof variable !== 'undefined' ? variable : '';
Also this will be helpful, when you try to declare the Global variable with instance assignment of the reference variable.
If you wanted to check variable shouldn't be undefined or null. Then perform below check.
When the variable is declared, and if you want to check the value, this is even Simple: and it would perform undefined and null checks together.
var values = variable ? variable : '';
It depends if you just care that the variable has been defined or if you want it to have a meaningful value.
Checking if the type is undefined will check if the variable has been defined yet.
=== null or !== null will only check if the value of the variable is exactly null.
== null or != null will check if the value is undefined or null.
if(value) will check if the variable is undefined, null, 0, or an empty string.
Try-catch
If variable was not defined at all (for instance: external library which define global variable is not yet loaded - e.g. google maps), you can check this without break code execution using try-catch block as follows (you don't need to use strict mode)
try{
notDefinedVariable;
} catch(e) {
console.log('detected: variable not exists');
}
console.log('but the code is still executed');
notDefinedVariable; // without try-catch wrapper code stops here
console.log('code execution stops. You will NOT see this message on console');
BONUS: (referring to other answers) Why === is more clear than == (source)
if( a == b )
if( a === b )
The highest answer is correct, use typeof.
However, what I wanted to point out was that in JavaScript undefined is mutable (for some ungodly reason). So simply doing a check for varName !== undefined has the potential to not always return as you expect it to, because other libs could have changed undefined. A few answers (#skalee's, for one), seem to prefer not using typeof, and that could get one into trouble.
The "old" way to handle this was declaring undefined as a var to offset any potential muting/over-riding of undefined. However, the best way is still to use typeof because it will ignore any overriding of undefined from other code. Especially if you are writing code for use in the wild where who knows what else could be running on the page...
if (typeof console != "undefined") {
...
}
Or better
if ((typeof console == "object") && (typeof console.profile == "function")) {
console.profile(f.constructor);
}
Works in all browsers
To contribute to the debate, if I know the variable should be a string or an object I always prefer if (!variable), so checking if its falsy. This can bring to more clean code so that, for example:
if (typeof data !== "undefined" && typeof data.url === "undefined") {
var message = 'Error receiving response';
if (typeof data.error !== "undefined") {
message = data.error;
} else if (typeof data.message !== "undefined") {
message = data.message;
}
alert(message);
}
..could be reduced to:
if (data && !data.url) {
var message = data.error || data.message || 'Error receiving response';
alert(message)
}
To check if a variable has been declared/set I did this dirty trick.
I haven't found a way to extract the code to a function, even with eval. Se this comment below for an explanation about why.
"use strict";
// var someVar;
var declared;
try {
someVar;
declared = true;
} catch(e) {
declared = false;
}
if (declared) {
console.log("someVar is declared; now has the value: " + someVar);
} else {
console.log("someVar is not declared");
}
The most robust 'is it defined' check is with typeof
if (typeof elem === 'undefined')
If you are just checking for a defined variable to assign a default, for an easy to read one liner
you can often do this:
elem = elem || defaultElem;
It's often fine to use, see: Idiomatic way to set default value in javascript
There is also this one liner using the typeof keyword:
elem = (typeof elem === 'undefined') ? defaultElem : elem;
Null is a value in JavaScript and typeof null returns "object"
Therefore, accepted answer will not work if you pass null values. If you pass null values, you need to add an extra check for null values:
if ((typeof variable !== "undefined") && (variable !== null))
{
// the variable is defined and not null
}
In the particular situation outlined in the question,
typeof window.console === "undefined"
is identical to
window.console === undefined
I prefer the latter since it's shorter.
Please note that we look up for console only in global scope (which is a window object in all browsers). In this particular situation it's desirable. We don't want console defined elsewhere.
#BrianKelley in his great answer explains technical details. I've only added lacking conclusion and digested it into something easier to read.
It is difficult to distinguish between undefined and null. Null is a value you can assign to a variable when you want to indicate that the variable has no particular value. Undefined
is a special value which will be the default value of unassigned variables.
var _undefined;
var _null = null;
alert(_undefined);
alert(_null);
alert(_undefined == _null);
alert(_undefined === _null);
you can use the typeof operator.
For example,
var dataSet;
alert("Variable dataSet is : " + typeof dataSet);
Above code snippet will return the output like
variable dataSet is : undefined.
I use two different ways depending on the object.
if( !variable ){
// variable is either
// 1. '';
// 2. 0;
// 3. undefined;
// 4. null;
// 5. false;
}
Sometimes I do not want to evaluate an empty string as falsey, so then I use this case
function invalid( item ){
return (item === undefined || item === null);
}
if( invalid( variable )){
// only here if null or undefined;
}
If you need the opposite, then in the first instance !variable becomes !!variable, and in the invalid function === become != and the function names changes to notInvalid.
My preference is typeof(elem) != 'undefined' && elem != null.
However you choose, consider putting the check in a function like so
function existy (x) {
return typeof (x) != 'undefined' && x != null;
}
If you don't know the variable is declared then continue with typeof (x) != 'undefined' && x != null;
Where you know the variable is declared but may not be existy, you could use
existy(elem) && doSomething(elem);
The variable you are checking may be a nested property sometimes. You can use prop || {} to go down the line checking existance to the property in question:
var exists = ((((existy(myObj).prop1||{}).prop2||{}).prop3||{})[1]||{}).prop4;
After each property use (...' || {}').nextProp so that a missing property won't throw an error.
Or you could use existy like existy(o) && existy(o.p) && existy(o.p.q) && doSomething(o.p.q)
These answers (aside from the Fred Gandt solution ) are all either incorrect or incomplete.
Suppose I need my variableName; to carry an undefined value, and therefore it has been declared in a manner such as var variableName; which means it's already initialized; - How do I check if it's already declared?
Or even better - how do I immediately check if "Book1.chapter22.paragraph37" exists with a single call, but not rise a reference error?
We do it by using the most powerful JasvaScript operator, the in operator.:
"[variable||property]" in [context||root]
>> true||false
It depends on the situation. If you're checking for something that may or may not have been defined globally outside your code (like jQuery perhaps) you want:
if (typeof(jQuery) != "undefined")
(No need for strict equality there, typeof always returns a string.) But if you have arguments to a function that may or may not have been passed, they'll always be defined, but null if omitted.
function sayHello(name) {
if (name) return "Hello, " + name;
else return "Hello unknown person";
}
sayHello(); // => "Hello unknown person"
if (variable === undefined) {}
works just fine, and only checks for undefined.
You could use a try...catch block like the following:
var status = 'Variable exists'
try {
myVar
} catch (ReferenceError) {
status = 'Variable does not exist'
}
console.log(status)
A disadvantage is you cannot put it in a function as it would throw a ReferenceError
function variableExists(x) {
var status = true
try {
x
} catch (ReferenceError) {
status = false
}
return status
}
console.log(variableExists(x))
Edit:
If you were working in front-end Javascript and you needed to check if a variable was not initialized (var x = undefined would count as not initialized), you could use:
function globalVariableExists(variable) {
if (window[variable] != undefined) {
return true
}
return false
}
var x = undefined
console.log(globalVariableExists("x"))
console.log(globalVariableExists("y"))
var z = 123
console.log(globalVariableExists("z"))
Edit 2:
If you needed to check if a variable existed in the current scope, you could simply pass this to the function, along with the name of the variable contained in a string:
function variableExists(variable, thisObj) {
if (thisObj[variable] !== undefined) {
return true
}
return false
}
class someClass {
constructor(name) {
this.x = 99
this.y = 99
this.z = 99
this.v = 99
console.log(variableExists(name, this))
}
}
new someClass('x')
new someClass('y')
new someClass('z')
new someClass('v')
new someClass('doesNotExist')
I prefer this method for it's accuracy and succinctness:
var x
if (x === void 0) {
console.log(`x is undefined`)
} else {
console.log(`x is defined`)
}
As has been mentioned in other comments and answers, undefined isn't guaranteed to be undefined. Because it's not a keyword, it can be redefined as a variable in scopes other than the global scope. Here's little example that demonstrates this nuance:
var undefined = 'bar'
console.log(`In the global scope: ${undefined}`)
function foo() {
var undefined = 'defined'
var x
if (x === undefined) {
console.log(`x === undefined`)
} else {
console.log(`x !== undefined`)
}
if (x === void 0) {
console.log(`x === void 0`)
} else {
console.log(`x !== void 0`)
}
}
foo()
See void for compatibility (supported in IE5!?!! Wow!).
I'm surprised this wasn't mentioned yet...
here are a couple of additional variations using this['var_name']
the benefit of using this method that it can be used before a variable is defined.
if (this['elem']) {...}; // less safe than the res but works as long as you're note expecting a falsy value
if (this['elem'] !== undefined) {...}; // check if it's been declared
if (this['elem'] !== undefined && elem !== null) {...}; // check if it's not null, you can use just elem for the second part
// these will work even if you have an improper variable definition declared here
elem = null; // <-- no var here!! BAD!
In ReactJS, things are a bit more complicated! This is because it is a compiled environment, which follows ESLint's no-undef rule since react-scripts#2.0.3 (released Oct. 1st, 2018). The documentation here is helpful to anyone interested in this problem...
In JavaScript, prior to ES6, variable and function declarations are hoisted to the top of a scope, so it's possible to use identifiers before their formal declarations in code....
This [new] rule [of ES6] will warn when it encounters a reference to an identifier that has not yet been declared.
So, while it's possible to have an undefined (or "uninitialized") variable, it is not possible to have an undeclared variable in ReactJS without turning off the eslint rules.
This can be very frustrating -- there are so many projects on GitHub that simply take advantage of the pre-ES6 standards; and directly compiling these without any adjustments is basically impossible.
But, for ReactJS, you can use eval(). If you have an undeclared variable like...
if(undeclaredvar) {...}
You can simply rewrite this part as...
if(eval('typeof undeclaredvar !== "undefined"')) {...}
For instance...
if(eval("false")) {
console.log("NO!");
}
if(eval("true")) {
console.log("YEAH!");
}
For those importing GitHub repositories into a ReactJS project, this is simply the only way to check if a variable is declared. Before closing, I'd like to remind you that there are security issues with eval() if use incorrectly.
For the if condition to work correctly, we have to use the keyword let for creating variables.
let name = undefined;
if (name) {
alert('valid')
};
NOTE: As per ECMAScript5.1, section 15.1.1.3, window.undefined is read-only.
Modern browsers implement this correctly. for example: Safari 5.1, Firefox 7, Chrome 20, etc.
Undefined is still changeable in: Chrome 14, ...
When I recently integrated Facebook Connect with Tersus, I initially received the error messages Invalid Enumeration Value and Handler already exists when trying to call Facebook API functions.
It turned out that the cause of the problem was
object.x === undefined
returning false when there is no property 'x' in 'object'.
I worked around the problem by replacing strict equality with regular equality in two Facebook functions:
FB.Sys.isUndefined = function(o) { return o == undefined;};
FB.Sys.containsKey = function(d, key) { return d[key] != undefined;};
This made things work for me, but seems to hint at some sort of collision between Facebook's JavaScript code and my own.
What could cause this?
Hint: It is well documented that undefined == null while undefined !== null. This is not the issue here. The question is how comes we get undefined !== undefined.
The problem is that undefined compared to null using == gives true.
The common check for undefined is therefore done like this:
typeof x == "undefined"
this ensures the type of the variable is really undefined.
It turns out that you can set window.undefined to whatever you want, and so get object.x !== undefined when object.x is the real undefined. In my case I inadvertently set undefined to null.
The easiest way to see this happen is:
window.undefined = null;
alert(window.xyzw === undefined); // shows false
Of course, this is not likely to happen. In my case the bug was a little more subtle, and was equivalent to the following scenario.
var n = window.someName; // someName expected to be set but is actually undefined
window[n]=null; // I thought I was clearing the old value but was actually changing window.undefined to null
alert(window.xyzw === undefined); // shows false
I'd like to post some important information about undefined, which beginners might not know.
Look at the following code:
/*
* Consider there is no code above.
* The browser runs these lines only.
*/
// var a;
// --- commented out to point that we've forgotten to declare `a` variable
if ( a === undefined ) {
alert('Not defined');
} else {
alert('Defined: ' + a);
}
alert('Doing important job below');
If you run this code, where variable a HAS NEVER BEEN DECLARED using var,
you will get an ERROR EXCEPTION and surprisingly see no alerts at all.
Instead of 'Doing important job below', your script will TERMINATE UNEXPECTEDLY, throwing unhandled exception on the very first line.
Here is the only bulletproof way to check for undefined using typeof keyword, which was designed just for such purpose:
/*
* Correct and safe way of checking for `undefined`:
*/
if ( typeof a === 'undefined' ) {
alert(
'The variable is not declared in this scope, \n' +
'or you are pointing to unexisting property, \n' +
'or no value has been set yet to the variable, \n' +
'or the value set was `undefined`. \n' +
'(two last cases are equivalent, don\'t worry if it blows out your mind.'
);
}
/*
* Use `typeof` for checking things like that
*/
This method works in all possible cases.
The last argument to use it is that undefined can be potentially overwritten in earlier versions of Javascript:
/* # Trollface # */
undefined = 2;
/* Happy debuging! */
Hope I was clear enough.
That's a bad practice to use the == equality operator instead of ===.
undefined === undefined // true
null == undefined // true
null === undefined // false
The object.x === undefined should return true if x is unknown property.
In chapter Bad Parts of JavaScript: The Good Parts, Crockford writes the following:
If you attempt to extract a value from
an object, and if the object does not
have a member with that name, it
returns the undefined value instead.
In addition to undefined, JavaScript
has a similar value called null. They
are so similar that == thinks they are
equal. That confuses some programmers
into thinking that they are
interchangeable, leading to code like
value = myObject[name];
if (value == null) {
alert(name + ' not found.');
}
It is comparing the wrong value with
the wrong operator. This code works
because it contains two errors that
cancel each other out. That is a crazy
way to program. It is better written
like this:
value = myObject[name];
if (value === undefined) {
alert(name + ' not found.');
}
From - JQuery_Core_Style_Guidelines
Global Variables:
typeof variable === "undefined"
Local Variables:
variable === undefined
Properties:
object.prop === undefined
var a;
typeof a === 'undefined'; // true
a === undefined; // true
typeof a === typeof undefined; // true
typeof a === typeof sdfuwehflj; // true
A). I never have and never will trust any tool which purports to produce code without the user coding, which goes double where it's a graphical tool.
B). I've never had any problem with this with Facebook Connect. It's all still plain old JavaScript code running in a browser and undefined===undefined wherever you are.
In short, you need to provide evidence that your object.x really really was undefined and not null or otherwise, because I believe it is impossible for what you're describing to actually be the case - no offence :) - I'd put money on the problem existing in the Tersus code.