Best way to check for "undefined" in javascript

Question

Possible Duplicates:
Detecting an undefined object property in JavaScript
How to check for undefined in javascript?

What is the most appropriate way to test if a variable is undefined in js? I've seen people use

if (window.myVariable) 

also

if (typeof(myVariable) != "undefined")

and

if (myVariable) //this throws an error if undefined, should this be in Try/Catch?

Answer 1

if (my_var == undefined){
    // ...
}

Answer 2

If you are interested in finding out whether a variable has been declared regardless of its value, then using the in operator is the safest way to go. Consider this example.

// global scope
var theFu; // theFu has been declared, but its value is undefined
typeof theFu; // "undefined"

But this may not be the intended result for some cases, since the variable or property was declared but just not initialized. Use the in operator for a more robust check.

"theFu" in window; // true
"theFoo" in window; // false

If you are interested in knowing whether the variable hasn't been declared or has the value undefined, then use the typeof operator.

if(typeof myVar != 'undefined')

The typeof operator is guaranteed to return a string. Direct comparisons against undefined are troublesome as undefined can be overwritten.

window.undefined = "omg";

"omg" == undefined // true

if(window.myVar) will also include all these falsy values, so it's not good:

false
0
""
NaN
null
undefined

Your third case - if (myVariable) should never throw an error.

Edit: The third case can only throw an error if it hasn't been declared at all, or you're using Object.defineProperty from ECMAScript 5th ed. where a property can have a backing function, and that backing function throws an error. For example.

// abc was never declared.
if(abc) {}

// or it's a property that can throw an error
Object.defineProperty(window, "myVariable", { 
    get: function() { throw new Error("W00t?"); }, 
    set: undefined 
});
if(myVariable) {}

The if line will throw an error now. Try it in Chrome.

Answer 3

check this http://stackoverflow.com/questions/27509/detecting-an-undefined-object-property-in-javascript

Answer 4

I use it as a function parameter and exclude it on function execution that way I get the "real" undefined. Although it does require you to put your code inside a function. I found this while reading the jQuery source.

undefined = 2;

(function (undefined) {
   console.log(undefined); // prints out undefined
   // and for comparison:
   if (undeclaredvar === undefined) console.log("it works!")
})()

Of course you could just use typeof though. But all my code is usually inside a containing function anyways, so using this method probably saves me a few bytes here and there.

Answer 5

Using typeof is the best way. It will work when the variable has never been declared, unlike any comparison with the == or === operators or type coercion using if. (undefined, unlike null, may also be redefined, making it unreliable for comparison).

if (typeof someUndeclaredVariable == "undefined") {
    // Works
}

if (someUndeclaredVariable === undefined) { 
    // Throws an error
}

Answer 6

I personally use

myVar === undefined

Warning: Please note that === is used over ==!

<rant> I do not buy the argument that undefined can be redefined. Sure it's true, but it's BS in my opinion to use that as an argument because just about ANYTHING in Javascript can be overwritten. Might as well not write any code at all because you "don't know what it does". How do I know Object, String, parseInt, etc. all are what they are? I don't, but there are assumptions one has to make. undefined is no different. </rant>

Answer 7

You need quotes if using typeof since it returns a string