<p>I've seen a couple of web pages say that <code>a = b || 'blah'</code> should assign <code>'blah'</code> to <code>a</code> if <code>b</code> is <code>undefined</code> or <code>null</code>.  But if I type that into Firebug or use it in code, it complains that <code>b</code> is not defined, at the list on FF3/win.  Any hints?</p>
<p>Edit: I'm looking for the case where <code>b</code> may not exist at all.  For example, a DOM node without an <code>id</code>.</p>

If b existed, and was false, null, etc, then it works in the way that you would expect. All you'll need to do is on the line above that, put
```
var b = null
```
;
This makes sense if you think about it. It basically does something like this...
```
```
a = function(){ if(b){return b;} else{ return 'blah' } }();
```
```
Note it is checking that the value of b is truthy... if b doesn't exist, you get an exception.
Regarding Undefined variables
"Undefined" in javascript doesn't mean 'variable doesn't exist'. It means "the value of the variable is the special value of
```
undefined
```
". Example:
```
```
alert(nosuchvariable);
=> throws exception

var somevariable; // note it's never assigned
alert(somevariable);
=> This alerts with 'undefined'
```
```
Regarding Checking if variables exist.
So if we try to read
```
b
```
and there is no such variable as b, we get an exception. If we're trying to find out if b is defined, then this isn't helpful.
You can see if global variables exist by checking the top-level
```
window
```
object. All global variables are actually just fields in the
```
window
```
object. Example:
```
```
foo = 'Hello';
alert( window.foo );
=> alerts 'Hello'
```
```
Because you know the window object already exists, you can check it's fields.
Checking for fields that don't exist in javascript will give you
```
undefined
```
and won't crash, so you can then do the coalesce, or put the
```
undefined
```
in a variable or whatever
For
local
variables (things declared with
```
var
```
),
you can't check for their existence.
they don't "live" anywhere in the way that global variables "live" in the window object, and any normal attempt to reference one will cause an exception: eg:
```
```
alert(a);
=> exception because a is meaningless
alert(d45pwiu4309m9rv43);
=> exception because that is equally meaningless
```
```
There is however one exception (that I know of, thanks J c in the comments), the
```
typeof
```
operator. If you try and get the type of something that doesn't exist, it
won't
crash, it will return
the string
```
"undefined"
```
.
This gives you a way of checking for non-existent local variables. eg:
```
```
if( typeof(djfsd) === "undefined" )
  alert('no such variable');
```
```
Regarding DOM elements that don't exist
There have been several comments mentioning DOM elements without ID's and so forth...
The fact that it's a DOM element isn't really relevant. Think of the DOM as a database or a file, and an element as a row in that database or word in that file. In order to
do
anything with it, you have to go searching through the database, find the right row, and pull it's data out. The data gets put into a javascript object. You then access it by manipulating that object, and maybe putting the object in a variable if you like. Example:
```
```
document.getElementById('foo');
```
```
this goes into the dom and looks for an element with an ID of 'foo'. If it finds one, it puts some information about that element into a javascript object, and then hands that object back to you. If it can't find the element, it will hand you back
```
null
```
, but all the normal rules still apply (you can stick the
```
null
```
in a variable, or whatever).
It doesn't affect the coalesce at all.