how to decide when to use array or objects in javascript

Question

I had an interview where the person asked me this question. Can anyone explains all the aspects so that i have a clear idea which structure to use when.

To be simple: Do you need named keys? Object! Don't? Array! The rest is opinion based. — eisbehr, Aug 30 '16 at 10:11
Here is a good read for you: http://www.metaltoad.com/blog/javascript-understanding-objects-vs-arrays-and-when-use-them — Dat Nguyen, Aug 30 '16 at 10:13
@JaromandaX thanks...actually he asked that both of them can do the same job at the end..Is there a striking difference i can put up which separates them totally. — Aman Seth, Aug 30 '16 at 10:20
Possible duplicate of: http://stackoverflow.com/questions/17295056/array-vs-object-efficiency-in-javascript — eisbehr, Aug 30 '16 at 10:21

score 0 · Accepted Answer · answered Aug 30 '16 at 11:23

Arrays:

Arrays provide order, Object's don't (at least not yet).
Arrays can be optimized (speed and even the memory footprint), when used as non sparse lists with items of the same type.
Arrays provide a bunch of functions to work with this data-structure

Objects:

Objects are more generic
Using the Object as a Dictionary: lookups are faster than iterating over an Array to find the right item.
return itemsById[id]
Code for Objects with the same hidden class can also be optimized.

Conclusion:

Arrays: use them if you think of the data as a list; same type improves performance but ain't a requirement.
To be more precise: lists have no gaps (missing indices), the data may have gaps, but the list shouldn't. If the list would have gaps, then the index has a meaning beyond just order, and it's probably a Dictionary with numeric keys.

Dictionaries: Plain Object used as a dictionary for the advantage of the fast lookup. Mentioned separate, because it's a different state of mind, having/dealing with a dictionary of items and dealing with an Object composing some properties.

Objects: A composition of properties of "some object". Like the body parts of a person. Or the "row" in a table of data: {id, firstName, lastName, ...}
Avoid enumerated properties. If you have sth. like {foo1, foo2, foo3, ...} foo is most likely a list of whatever, and should be built as such (just makes your life easier).
unless this object is a dictionary

how to decide when to use array or objects in javascript

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