Javascript arrays?

Question

This may be completely stupid but given I type this into my browser console:

var arr = [];
arr.item = 'val';
console.log( arr );
arr;

What is the arr; syntax doing behind the scenes? I'm assuming console.log( arr ); is iterating all the properties of the arr object but what is arr; doing? Also does [] tell me I'm dealing with an object of type array and {} tells me I'm dealing with an object of type object literal? So [ item: 'val' ] is an object with a prototype of array and { item: 'val' } is an object of prototype object literal?

Edit: Another way to ask this might be why are console.log and arr; different?

Answer 1

Sorry, started to type the comment and realized it was actually a full answer.
console.log yields an implementation dependent string representation of the argument. It does not iterate the properties of an object in most I've seen. String serializtions using the default toString method look like [object Foo] where 'Foo' is the internal class property of the object. You can see this for arrays by using Object.prototype.toString.call([]);.

The last line on the other hand, is just the value. How that gets represented as an evaluated expression in your browser console is implementation-dependent, but for all intents and purposes it does nothing and there is no reason you would want to write that.

Note that adding a property to an array with the . operator is almost never what you want: it does not add to the contents of the array but treats the array as an object. Iterating the indicies of the array will not yield that property, nor will its methods such as .map include it. Use .push. If you want string-keyed properties, use an object.