The use of undefined variables in if-statements

Question

This snippet results in a JavaScript runtime error: (foo is not defined)

if (foo) {
    // ...
}

I have to define foo first, like so:

var foo = foo || null // or undefined, 0, etc.

... And only then can I do:

if (foo) {
    // ...
}

Why is that?


Update:

This was somewhat of a brainfart on my side of things: 'fcourse you can't access a variable which is not allocated.

Fun stuff that you can do a typeof() on an undefined variable thou. I'm gonna accept miccet's answer since I think it's the most elegant solution.

Answer 1

I sense you are asking because you are aware that javascript seems to allow undefined variables in some situations (ie no runtime errors) and not in others.

The reasoning is as follows: javascript always throws an error on checking undefined variables, but never throws an error on checking undefined properties, as long as you only use one level of indirection.

Example:

// throws runtime error
if(foo) {
    // ...
}

// does not throw runtime error
if(window.foo) {
    // ...
}

// does not throw runtime error
var obj = {};
if(obj.foo) {
    // ...
}

// throws runtime error
if(obj.foo.bar) { // going two levels deep, but foo is undefined
    // ...
}

Hope that clears it up a bit.

Answer 2

You'll have to define it, to be able to check it for a value. In this case you're checking weather it's true. This variable is obviously not set to anything at all, same as null in C# and Nothing in VB for example.

If you must, debugging issues or whatever, you could check if the variable is undefined like this:

if (typeof(variable) == "undefined")

Answer 3

That would be because you're now defining it with:

var foo = foo || null

Why don't you define it in the first place? That seems pretty straightforward to me (unless I'm missing something).

Using a variable before it's created (or set to something) is bad programming practice, and should be avoided. My advice is to not use that trick to ensure it is set to something but to track down the logic error and fix it.