Q: Differentiating between arrays and "hashes" in Javascript In order to make the syntax for one of my functions nicer, I need to be able to tell whether a specific parameter is an array or "hash" (which I know are just objects).
Typeof doesn't work, because they both return the same thing
typeof {foo:"bar"} // Object
typeof ["foo","bar"] // Object
So how would I differentiate between the two?
I know this works, but I'm hoping there's a nicer way
({foo:"bar"}).constructor // Object()
(["foo","bar"]).constructor // [ undefined ]
EDIT
Ah, it seems [ undefined ] in firebug is the same thing as Array. Kind of weird.

A: something instanceof Array works fine within a single document, but will fail if you start passing arrays between different windows, because the Array from one window is a different object from Array on another. If you have no intention of doing cross-window-scripting (and in general, it's worth avoiding) I would recommend sticking with this.
If you need cross-window support, things are a bit more complex. In the future the story is simple, as ECMAScript Fifth Edition defines a function to do exactly this:
Array.isArray([1]);   // -> true

You should use this functionality where available as it's the only reliable and standards-endorsed way. However, many of today's browsers don't support it yet.
Where it isn't available you have to rely on Object#toString serialisation, which is ugly and slightly dodgy. Although it will in general work reliably with today's browsers, there are imaginable cases where it might not (primarily to do with host objects).
You can hack this fallback method into Array on browsers that don't support it, and then use Array.isArray at all times:
if (!('isArray' in Array)) {
    Array.isArray= function(o) {
        return Object.prototype.toString.call(o)==='[object Array]';
    };
}

As for constructor, never use it. It's not available everywhere and it doesn't do what you think. Using it is almost always a mistake.

A: I think the most elegant way is to simply use the instanceof operator:
if (myVar instanceof Array)
    doSomething();

examples: 
[] instanceof Array           // true
{} instanceof Array           // false
{length:100} instanceof Array // false
null instanceof Array         // false

Edit:
Be aware that it will fail when testing an object from another iFrame (see answers by @galambalazs and @bobince)

A: You could check the length property as SLaks suggested, but as soon as you pass it a function object you'll be surprised, because it in fact has a length property. Also if the object has a length property defined, you'll get wrong result again.
Your best bet is probably:
function isArray(obj) {
    return Object.prototype.toString.call(obj) === "[object Array]";
}

jQuery uses it, and a "couple of" other people... :)
It is more fail proof than the instanceof way. The method is also suggested by the following article:
'instanceof' considered harmful (or how to write a robust 'isArray') (@kagax) 
Another thing to add that this function is almost identical to the Array.isArray function in ES 5 spec:

15.4.3.2   Array.isArray ( arg )
  
  
*
  
*If Type(arg) is not Object, return
  false. 
  
*If the value of the [[Class]]
  internal property of arg is "Array",
  then return true. 
  
*Return false.
  


A: Check for the length property:
"length" in {foo:"bar"}   //false
"length" in ["foo","bar"] //true


A: This typeof function is used to correct the array/object clash for the typeof operator, and the null/object clash. It does not however work across frames or across windows. See the source for a more advanced version that works across these.
function typeOf(value) {
    var s = typeof value;
    if (s === 'object') {
        if (value) {
            if (value instanceof Array) {
                s = 'array';
            }
        } else {
            s = 'null';
        }
    }
    return s;
}

Source: Douglas Crockford's Javascript site
