The following two different code snippets seem equivalent to me:
var myArray = Array();
myArray['A'] = "Athens";
myArray['B'] = "Berlin";
and
var myObject = {'A': 'Athens', 'B':'Berlin'};
because they both behave the same, and also typeof(myArray) == typeof(myObjects) (both yield 'object').
Is there any difference between these variants?
Virtually everything in javascript is an object, so you can "abuse" an Array object by setting arbitrary properties on it. This should be considered harmful though. Arrays are for numerically indexed data - for non-numeric keys, use an Object.
Here's a more concrete example why non-numeric keys don't "fit" an Array:
var myArray = Array();
myArray['A'] = "Athens";
myArray['B'] = "Berlin";
alert(myArray.length);
This won't display '2', but '0' - effectively, no elements have been added to the array, just some new properties added to the array object.
Me thinks, me too metaphorical and cryptic with previous answer. Clarification follows.
An instance of Array, Boolean, Date, Function, Number, RegExp, String is an Object but enhanced with methods and properties specific to each type. For example, an array has a predefined length property while generic objects do not.
javascript:alert([].length+'\n'+{}.length)
displays
0 undefined
Intrinsically, the FF Gecko interpreter also distinguishes between Arrays and generic Objects with distinct differences evaluating language constructs.
javascript:
ra=[ "one", "two", "three"]; ra.a=4;
ob={0:"one", 1:"two", 2:"three"}; ob.a=4;
alert(
ra +"\n\n"+
ob +"\n\n"+
ra.toSource() +"\n\n"+
ra.a +"\t .toSource() forgot me! \n\n"+
ra.length +"\t and my length! \n\n"+
ob.toSource());
ps=""; for(i in ra)ps+=i+" "; alert(ps); /* NB .length is missing! */
ps=""; for(i in ob)ps+=i+" "; alert(ps);
displaying
one,two,three
[object Object]
["one", "two", "three"]
4 .toSource() forgot me!
3 and my length!
({0:"one", 1:"two", 2:"three", a:4})
and 0 1 2 a and 0 1 2 a.
Regarding the statement that all objects are functions:
It is neither syntactically nor semantically correct to use an arbitrary object instance as a function like 123() or "abc"() or []() or {}() or obj() where obj is any type other than Function, so an arbitrary object INSTANCE is not a Function. However, given an object obj and it's type as Array, Boolean, Date, ..., how did obj come to be as an Array, Boolean, Date, ...? What is an Array, Boolean, Date, ...?
javascript:
alert([ArrayThe {}-notation is just syntactical sugar to make the code nicer ;-)
JavaScript has many similar constructs like the construction of functions, where function() is just a synonym for
var Func = new Function("<params>", "<code>");