Question

What is the quickest and cleanest way to test if a var holds immutable data (i.e. is string, number, boolean, null, undefined)?

E.g. When var test is mutable, the following is legal:

var test = {}; 
//the next 2 lines work because `test` has mutable data
test.someFun = function () { alert(this); };
test.someFun(); 

but when var test is immutable, it’s illegal:

var test = "string"; 
//the next 2 lines are invalid, as `test` is a primitive
test.someFun = function () { alert(this); };
test.someFun(); 

Answer 1

var test = {};
if(test instanceof Object){
  test.someFun = function () { alert(this); };
  test.someFun(); 
}

jball pointed out the following:

For the instanceof check to work, the
left-hand side object should be from
the same scope as the right-hand side
object. This is because even the
common JavaScript objects have
different copies in different scopes,
i.e. window1.String !== window2.String.
(Quote MDN)

Seems a valid point to me. So if you want to do this check across different scopes you will need to use the workaround:

var test = {};
if(test && (typeof test == 'object' || typeof test == 'function')){
  test.someFun = function () { alert(this); };
  test.someFun(); 
}

I would try to avoid the use of iframes though.
The null check is needed because typeof(null) is equal to 'object'.

EDIT:

box9 and jball did some performance testing on all the methods proposed (+1 for that). I used Chrome, Firefox and Internet Explorer. The result are as expected. These two methods are the fastest. On Chrome and Firefox the typeofversion is the fastest. On Internet Explorer the instanceof version was the fastest. But the difference in speed between these two methods were minor. Personally I’d still go with the instanceof method if I could because it’s cleaner.